Method of detecting position of mark on substrate, position detection apparatus using this method, and exposure apparatus using this position detection apparatus

ABSTRACT

In a method of detecting a position of a position detection grating mark formed with a small step structure on a surface of a flat object, an illumination light beam is irradiated on the grating mark at a predetermined incident angle. The illumination light beam includes a plurality of coherent beams having n (n≧3) different wavelengths λ 1 , λ 2 , λ 3 , . . . , λ n . The n wavelengths are set to approximately satisfy the following relation within a range of about ±10%: 
     
       
         (1/λ 1 −1/λ 2 )−(1/λ 2 −1/λ 3 )= . . . =(1/λ n−1 −1/λ n ) 
       
     
     where the wavelengths have a condition λ 1 &lt;λ 2 &lt;λ 3  . . . &lt;λ n . Photoelectric detection of a change in light amount of a diffracted light component generated from the grating mark in a specific direction is performed upon irradiation of the illumination light beam having the n wavelength components. The position of the grating mark is determined based on a signal obtained in the photoelectric detection. A position detection apparatus using the above method and an exposure apparatus using this position detection apparatus are also disclosed.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of application Ser. No. 09/388,352 filedSep. 1, 1999, now U.S. Pat. No. 6,242,754, which is a division of Ser.No. 08/719,063 filed Sep. 24, 1996, now U.S. Pat. No. 6,034,378 issuedMar. 7, 2000, which is a continuation-in-part of Ser. No. 08/593,935filed Jan. 30, 1996 (abandoned).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a technique for relative positioningbetween a mask pattern and a photosensitive substrate, which techniqueis applied to an exposure apparatus used in a photolithography processfor exposing the mask pattern onto the photosensitive substrate inmanufacturing, e.g., semiconductor elements and, more particularly, to atechnique for detecting the mark pattern on the photosensitivesubstrate.

2. Related Background Art

In the photolithography process for manufacturing, e.g., semiconductorelements, liquid crystal display elements, or thin-film magnetic heads,an exposure apparatus is used to transfer an image of a photomaskhereinafter) having a transfer pattern onto a photoresist-coated wafer(or a photosensitive substrate such as a glass plate) in accordance witha projection exposure method through a projection optical system orproximity exposure method.

In this exposure apparatus, positioning (alignment) between a reticleand a wafer must be performed with high precision prior to exposure. Aposition detection mark (alignment mark) obtained by exposure, transfer,and etching in the previous process is formed on the wafer. The accurateposition of the wafer (i.e., a circuit pattern on the wafer) can bedetected by detecting the position of this alignment mark.

In recent years, there has been proposed a method of forming aone-dimensional or two-dimensional grating mark on a wafer (or areticle), projecting two coherent beams symmetrically inclined in thepitch direction on the grating mark, and causing two diffracted lightcomponents generated from the grating mark in the same direction tointerfere with each other to detect the position and offset of thegrating mark in the pitch direction, as disclosed in Japanese PatentApplication Laid-open Nos. 61-208220 (corresponding U.S. Pat. No.4,828,392; to be referred to as reference (A) hereinafter), 61-215905(corresponding U.S. Pat. No. 4,710,026; to be referred to as reference(B) hereinafter), and the like. Reference (A) discloses a homodynescheme in which two symmetrical coherent beams have the same frequency,while reference (B) discloses a heterodyne scheme in which apredetermined frequency difference is present between two symmetricalcoherent beams.

Heterodyne schemes in which position detection devices of these schemesare applied to a TTR (Through-The-Reticle) alignment system and a TTL(Through-The-Lens) alignment scheme are proposed in Japanese PatentApplication Laid-open Nos. 2-227602 (corresponding U.S. Ser. No. 483,820filed on Feb. 23, 1990, now U.S. Pat. No. 5,489,968 issued Feb. 6, 1996;to be referred to as reference (C) hereinafter), 3-2504 (correspondingU.S. Pat. No. 5,118,953; to be referred to as reference (D)hereinafter), and the like. In the heterodyne schemes disclosed in thesereferences (C) and (D), an He—Ne laser beam is simultaneously incidenton two acousto-optic modulators (AOMs), and the AOMs are driven byhigh-frequency drive signals (one drive signal: 80 MHz; the other drivesignal: 79.975 MHz) having a frequency difference of, e.g., about 25kHz, thereby imparting the frequency difference of 25 kHz to diffractedbeams emerging from these AOMs. These two diffracted beams serve as apair of incident beams for irradiating a grating mark on a wafer orreticle at a predetermined crossing angle.

In the heterodyne scheme, the frequency difference (25 kHz) between thetwo incident beams is given as a reference AC signal, a phase differencebetween the reference AC signal and a signal obtained byphotoelectrically detecting an interference light beam (beat light beam)of two diffracted light components generated from the grating mark ismeasured, and this phase difference is detected as a position offset(positional deviation) amount from the reference point in the pitchdirection of the grating mark.

According to the heterodyne scheme described above, when the twoincident beams for illuminating the grating mark have bettermonochromaticity, the detection precision in position offset, i.e., aresolution can increase. Position detection and positioning on thenanometer order can be performed. Excellent monochromaticity in the twoincident beams indicates that the phase on the wavelength order betweenvarious diffracted light components generated by the grating mark tendsto be sensitively changed by asymmetry of the grating marks, a resistlayer, and the like.

An influence of the resist layer cannot be inevitably avoided in waferalignment in an exposure apparatus. Unless a special technique forlocally removing a resist from a mark portion is used, or unless anoptical mark detection technique is given up, this problem is leftunsolved.

A heterodyne scheme capable of reducing the influence of the resistlayer and an influence of asymmetry in the sectional shape of the marksand allowing more accurate position detection is proposed in JapanesePatent Application Laid-open No. 6-82215 (corresponding U.S. Ser. No.091,501 filed on Jul. 14, 1993; to be referred to as reference (E)hereafter). According to a technique disclosed in this reference (E), aplurality of beams having different wavelengths, or a white beam isused, and two diffracted beams obtained upon irradiation of theplurality of beams or the white beam on a stationary diffraction gratingare incident on the first AOM. 0th-, +1st- and −1st-order beamsdiffracted by the first AOM are relayed to cross each other in thesecond AOM, thereby obtaining a pair of incident beams having the firstwavelength and a pair of incident beams having the second wavelength.These two pairs of incident beams are simultaneously projected onto thegrating mark on the wafer.

In this case, an interference beat light beam generated by the gratingmark and photoelectrically converted include the first wavelengthcomponent and the second wavelength component. These components arephotoelectrically detected in the form of a sum as a light amount on thelight-receiving surface of a photoelectric element. For this reason, themutual phase differences of the interference beat light beams of therespective wavelength components caused by the influence of thethin-film interference of the resist layers and the influence ofasymmetry in the sectional shape of the marks can be averaged in termsof light intensity. Therefore, more accurate position detection can beperformed.

Regardless of the homodyne and heterodyne schemes, a light sourcesuitable for obtaining a multi-wavelength illumination light beam isgenerally selected from light sources having a high coherency and asufficiently large light intensity, such as a gas laser light source ora semiconductor laser light source. For this reason, wavelengthsselected for a multi-wavelength light beam in the conventionalarrangement are determined such that the oscillation center frequency isshifted by an appropriate amount, e.g., 20 nm to 40 nm from practicallight sources (e.g., laser light sources having excellent records ofperformance).

The surfaces of the grating marks on the wafer are coated with a resistlayer having an almost predetermined thickness (e.g., about 0.5 μm to1.5 μm). This thickness varies on a given central value. In addition,the thickness of the resist layer varies depending on the positions onthe wafer. The sectional shape (small step differences of the gratinggrooves) of the grating marks slightly changes depending on thepositions on the wafer in accordance with a wafer process. The totalamplitude reflectance of the portion including the grating marks and theresist layer on their surfaces often greatly varies with respect to anillumination light beam which is converted as a multi-wavelengthillumination light beam with a specific wavelength component.

Wavelength selection for forming a multi-wavelength illumination lightbeam cannot always be optimized so as to achieve high-precision positiondetection for a grating mark whose amplitude reflectance can greatlyvary.

Assume that an illumination light beam used for position detection isformed into a beam having a plurality of wavelengths or a beam having apredetermined wavelength bandwidth, and that interference light beamsincluding a plurality wavelength components generated by the gratingmark are simultaneously received by a single photoelectric element. Inthis case, if the illumination light beam includes a high-intensitywavelength component, the interference light beam from the grating markis increased at this wavelength component, thus often posing a problemto obtain an averaging effect. In addition, even if the intensities ofthe wavelength components in the illumination light beam are identicalto each other, large differences may occur for the wavelength componentsof the interference light beam from the grating mark depending on thesurface state (e.g., the irregularity in thickness of the resist and thedegree of asymmetry of the grating marks) on the photosensitivesubstrate such as a wafer.

Even if an interference light beam having a plurality of wavelengthcomponents generated by the grating mark is received by a singlephotoelectric element, high precision of position detection cannotalways be obtained depending on the surface state of the substrate.

A heterodyne system in which the influence of a resist layer and theinfluence by the deformation of the shape (such as the asymmetry of thecross-sectional shape) of a grating mark are reduced to thereby makemore accurate position detection possible is also proposed by U.S. Pat.No. 5,160,849 (H).

In this publication (H), there is disclosed a technique of finding afirst position offset amount of the grating mark measured on the basisof the photoelectric signal of the interference beat light of ±1st-orderdiffracted lights created perpendicularly from a grating beam and asecond position offset amount of the grating mark measured on the basisof 0-order diffracted light and ±2nd-order diffracted lights createdfrom the grating beam, and selecting one of the two positional deviationamounts.

Also in a position detecting system of the homodyne type, as disclosedin Japanese Patent Application Laid-open No. 61-208220 (I), there isproposed an attempt to know the asymmetry of a diffraction grating bycomparing the magnitudes of the light intensities of a plurality ofhigh-order diffracted lights created from a diffraction grating on asubstrate.

However, in the system of the publication (I) wherein as in the priorart, a variation in the distribution by the each of the intensities ofhigh-order diffracted lights created from a grating mark by theapplication of an illuminating beam of a single wavelength is presumed,a number of data bases for making various conditions such as the opticalnon-uniformity of a resist layer covering the grating mark and theasymmetry of the cross-sectional structure of the grating mark and thevariation in the distribution of the intensities of the high-orderdiffracted lights correspond to each other become necessary.

Also, in the system as disclosed in the publication (H) wherein one ofthe first position offset amount measured on the basis of the coherentlight of 1st-order diffracted components from the grating mark and theposition offset amount measured on the basis of the coherent lights of0-order and 2nd-order diffracted components, there has been the problemthat the deterioration of the position offset detection accuracy by theoptical non-uniformity of the resist layer and the influence of theasymmetry of the cross-sectional structure of the grating mark cannot besynthetically suppressed (the improvement in the accuracy becomeslimited).

Also, in connection with the technique of the publication (H), it wouldoccur to mind to average the measured first position offset amount andsecond position offset amount with weight given thereto in conformitywith the intensities (the degrees of modulation) of the respectiveinterference lights, but it may sometimes be impossible to attain animprovement in accuracy simply by the averaging having added thereto thesimple weight conforming to the degrees of modulation of theinterference lights from the grating mark on a wafer. This is consideredto be caused by the fact that the amplitude reflectance of the surfaceportion of the wafer including the grating mark for the illuminatingbeam changes variously for each wafer and at each location (shot area)on the wafer.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide aposition detection method and apparatus almost free from the influenceof a surface state of a periodic grating pattern (grating marks) formedon a substrate such as a wafer.

It is another object of the present invention to provide a positiondetection method and apparatus capable of adaptively changing the timecoherence of a multi-wavelength illumination light beam with changes insectional shape (e.g., a groove depth) of grating marks and thickness ofa resist when an illumination light beam for detecting the grating markon a substrate is formed as a multi-wavelength light beam using coherentbeams having relatively narrow spectral widths radiated from a pluralityof light sources.

It is still another object of the present invention to provide aprojection exposure apparatus capable of generating an incoherentmulti-wavelength illumination light beam using a plurality of coherentlight sources and using this illumination light beam for alignment ofgrating marks on a photosensitive substrate.

It is still another object of the present invention to solve theconventional problems in photoelectric detection described above and toprovide a position detection method or apparatus almost free from theinfluence of a surface state of a substrate such as a wafer.

It is still another object of the present invention to provide aposition detection method or apparatus almost free from the influence ofa light intensity difference between wavelength components even if agrating pattern (mark) is illuminated with an illumination light beamincluding a plurality of wavelength components.

It is still another object of the present invention to provide ahigh-precision positioning (alignment) apparatus which reduces a gratingpattern position measurement error depending on the state of a substratesurface when the position of a grating pattern is to be measured uponirradiation of the illumination light beam having a plurality ofwavelength components on the substrate.

Further, the present invention has as an object thereof to provide aposition detecting method or apparatus which is hardly affected by theasymmetry (a variation in the amplitude reflectance) of thecross-sectional structure of a grating pattern (mark) even when thegrating pattern is illuminated with an illuminating light beam of asingle wavelength.

Furthermore, the present invention has as an object thereof to provide ahighly accurate alignment apparatus which is reduced in the positionmeasurement error of a grating pattern depending on the state of thesurface of a substrate when the position of the grating pattern ismeasured by the use of an illuminating light beam of a singlewavelength.

In order to achieve the above objects of the present invention, thepresent invention is applied to a method or apparatus in which a pair ofcoherent light beams are incident on a position detection grating mark(MG) at incident angles symmetrical, with respect to the pitch directionof grating marks, the grating mark (MG) being formed with a small stepdifference structure on the surface of a plate-like object (W) such as asemiconductor wafer or glass plate, and the position of the grating markin the periodic direction of grating marks is detected on the basis ofsignals obtained by photoelectrically detecting changes in light amountsof diffracted light components from the grating mark in specificdirections.

According to the present invention, coherent light beams (±LF) forsymmetrically irradiating a grating mark are formed into n pairs oflight beams (±D_(1n)) having n (n=3 or more) different wavelengthsλ₁,λ₂, λ₃, . . . , λ_(n), and when the magnitudes of the wavelengthssatisfy λ₁<λ₂<λ₃, . . . , <λ_(n), the n wavelengths or a plurality ofcoherent light sources (LS1, LS2, LS3) are selected to approximatelysatisfy the following relation within the range of about ±10%:

(1/λ₁−1/λ₂)=(1/λ₂−1/λ₃)= . . . =(1/λ_(n−1)−1/λ_(n))

In a preferable arrangement of the present invention, the two lightbeams constituting a pair of multi-wavelength coherent light beams areheterodyned to have a predetermined frequency difference between them. Alight beam obtained by intensity-modulating, with a beat frequency, amutual interference light beam of two first-order diffracted lightcomponents generated from the grating mark (MG) in a directionperpendicular to the grating pitch direction is photoelectricallydetected.

In a preferred application of the present invention, a multi-wavelengthposition detection apparatus is incorporated in a projection exposureapparatus as a TTR, TTL, or off-axial alignment means (mark detectionmeans).

In order to solve the problem on photoelectric detection, the presentinvention is applied to a method of projecting an illumination lightbeam on a diffraction grating (MG) formed on a substrate (a wafer W or afiducial mark plate FG) subjected to position detection, andphotoelectrically detecting diffracted light components from thediffraction grating (MG), thereby detecting the position of thesubstrate. First, (a) illumination beams (e.g., beams ±D₁₁ and ±D₂₂diffracted by a reference grating RG) having different wavelengthcomponents (λ₁ and λ₂) are projected on the diffraction grating (MG) togenerate a plurality of diffracted beams having the respectivewavelength components from the diffraction grating (MG); and (b) a firstinterference beam formed by interference between two diffracted beamshaving an order difference (+1st and −1st orders; or 0th and 2nd orders)and having a first wavelength component (λ₁) of the plurality ofgenerated diffracted beams is received by a first photoelectric element,and a second interference beam formed by interference between twodiffracted beams having an order difference (+1st and −1st orders; or0th and 2nd orders) and having a second wavelength component (λ₂) of theplurality of generated diffracted beams is received by a secondphotoelectric element. Subsequently, (c) first position information(ΔλX₁) associated with the periodic direction of the diffraction gratingis calculated by a circuit unit (CU₃) on the basis of a photoelectricsignal (I_(m1)) from the first photoelectric element, and secondposition information (ΔX₂) associated with the periodic direction of thediffraction grating (MG) is calculated by a circuit unit (CU₄) on thebasis of a photoelectric signal (I_(m2)) from the second photoelectricelement. Finally, (d) the weighted mean of the first positioninformation and the second position information is calculated by acircuit unit (CU₅) by changing weighting factors in accordance with theamplitude value of the photoelectric signal from the first photoelectricelement and the amplitude value of the photoelectric signal from thesecond photoelectric element, thereby confirming the position of thesubstrate on which the diffraction grating is formed.

Positioning, i.e., position measurement marks formed on the surface of awafer or the like are generally formed on the surface with a small stepdifference. However, these marks have slight asymmetry depending on thewafer process such as etching and sputtering in the semiconductorprocessing or a coating irregularity in the photoresist layer. Thisasymmetry causes degradation of accuracy of mark position detection.

In the interference alignment method of photoelectrically detecting themutual interference light beam of the two diffracted light componentsgenerated by the grating mark and utilizing the resultant photoelectricsignal, the asymmetry of the grating marks causes asymmetry of theamplitude reflectances of the marks themselves, resulting in degradationof position detection precision. More specifically, if a differenceoccurs in the depths of the bottoms of the grooves of the linesconstituting the grating marks or the thickness of the resist layerlocally varies, the absolute values and phases of the amplitudereflectances of the marks themselves become asymmetrical in accordancewith the changes in depth of the bottom of the groove and thickness ofthe resist. As a result, the intensity and phase of the positive-orderdiffracted light component generated from the grating mark in the rightdirection relative to the zero order light are different from those ofthe negative-order diffracted light component generated in the leftdirection relative to the zero order light. In this case, the intensitydifference does not almost cause degradation of position detectionprecision, but the change in phase greatly affects the positiondetection precision.

The simulation results of position detection precision in the heterodynescheme using an illumination light beam having a single wavelength as inthe conventional case will be described with reference to FIGS. 1 and 2.This simulation is performed under an assumption that two coherentincident beams having a predetermined frequency difference areirradiated from two symmetrical directions on a grating mark MG on awafer covered with a resist layer PR, as shown in FIG. 2, and theresults are obtained by observing the state (e.g., an amplitude and aphase) of a mutual interference beam, i.e., an interference beat lightbeam, of the ±1st-order diffracted light components generated from thegrating mark MG in a direction perpendicular to the surface of thegrating mark MG while the wavelength is changed.

FIG. 2 illustrates a one-dimensional grating MG on a wafer or the likewhich is assumed in the simulation and the enlarged section of a gratingportion with the resist layer PR coated on the surface of the grating.In this case, a pitch Pmg of the grating MG was set to be 8 μm, a dutywas set to be 1:1, the step difference (depth) T₂ of a groove was set tobe 0.7 μm, and an asymmetry of 0.1% having a taper (inclination) ΔS inthe pitch direction was set in the bottom portion of the grating MG. Athickness T₁ of the resist layer PR which covered the grating MG fromthe surface of the top portion of the grating MG was set to be 0.9 μm,and a recess amount ΔT on the resist layer surface corresponding to theposition of each bottom portion of the grating MG was set to be ΔT≅0.3T₂ (0.21 μm). The grating structure shown in FIG. 2 is called a gratinghaving an asymmetrical amplitude reflectance.

FIG. 1 is a graph obtained when a wavelength λ (μm) of an illuminationlight beam or an interference light beam obtained by synthesizing±1st-order diffracted light components is plotted along the abscissa,and the relative amplitude of a change (AC component) of a signalcorresponding to a change in light amount of the interference light beamand a position detection error amount (μm) are plotted along theordinate. In the simulation results shown in FIG. 1, the conditions forthe grating mark structure and the resist layer in FIG. 2 were set suchthat the wavelength λ for outputting the zero AC component, i.e., onlythe DC component of the photoelectric signal corresponding to theinterference light beam received by the heterodyne scheme was 0.663 μmas the wavelength of the He—Ne laser.

As can be apparent from the above description, when a laser beam havinga wavelength of 0.663 μm is used, it is found that the mark positiondetection error near (about ±20 nm) this wavelength becomes very large.This is natural in the heterodyne scheme. If the AC componentcorresponding to the beat frequency is not included in a photoelectricsignal subjected to phase difference measurement, the phase differencemeasurement is impossible. This is also true for homodyne positiondetection under the same conditions as described above for the gratingmark structure and the resist layer.

The amplitude reflectance of the grating mark itself also greatly variesdepending not only on the depth of the mark and the thickness of theresist but also the wavelength components of an illumination light beam.For this reason, the variation in amplitude reflectance of the gratingmark is dependent on time coherence of the illumination light beam.

According to the first aspect of the invention for solving the problemon the illumination light beam side, to reduce the temporal coherence ofthe illumination light beam within a practical range with respect to theactual step difference structure of the marks and the actual state ofthe thickness of the resist layer and to obtain an incoherentillumination light beam, an assumption is given such that a plurality ofcoherent light beams having three or more different center wavelengthsλ₁, λ₂, and λ₃, . . . , λ_(n) are used to obtain multi-wavelength lightbeams.

According to the first aspect, as the basic condition for incoherence,when sets of coherent light beams adjacent to each other in the order ofwavelengths are taken into consideration, the respective wavelengths aredetermined such that a difference between wave number values(1/wavelength) of two light beams obtained for each set falls within theerror range of about ±10%.

The temporal coherence of a multi-wavelength illumination light beamobtained using three coherent light beams (the wavelength widths arevery narrow) having center wavelengths λ₁=0.633 μm, λ₂=0.690 μm, andλ₃=0.760 μm, respectively is simulated. The results are shown in FIG. 3.The thickness (μm) of a resist layer formed on a silicon substrate isplotted along the abscissa of FIG. 3, while the reflectance obtained asan intensity sum of reflected light components having a wavelengthgenerated by the silicon substrate upon irradiation of amulti-wavelength illumination light beam is plotted along the ordinateof FIG. 3. The amplitudes of reflectance variations with changes inresist thickness represent the temporal coherence of the illuminationlight beam. This temporal coherence can also be obtained by Fouriertransform of the spectral distribution of the illumination light beam.

According to the simulation results in FIG. 3, the variation amplitudesof the reflectances are found to be small and the temporal coherence isalso found to be small when the resist thicknesses (or mark stepdifferences) fall within the range of 0.5 μm to 1.7 μm. That is, analmost incoherent state is obtained. In this manner, the coherence canbe reduced within the range of specific optical thicknesses and smallstep differences (i.e., the resist thicknesses fall within the range of0.5 μm to 1.7 μm) because the basic conditions representing therelations of the wavelengths used to form a multi-wavelength light beamare satisfied within a deviation of ±10%.

More specifically, if the wavelengths λ₁, λ₂, and λ₃ are given as 0.663μm of the He—Ne laser beam, 0.690 μm of the beam of a commerciallyavailable semiconductor laser, and 0.760 μm of the beam of anothercommercially available semiconductor laser, respectively, the followingrelations are established:

Δλ₁₂=1/λ₁−1/λ₂=0.1305

Δλ₂₃=1/λ₂−1/λ₃=0.1335

and their deviation is given as Δλ₂₃/Δλ₁₂=1.023 (an error of 2.3%). Whena multi-wavelength illumination light beam is obtained from three lightbeams having wavelengths falling outside the above ranges, it isdifficult to obtain a good incoherent state with a small variationamplitude of the reflectance within a specific range.

Condition Δλ₁₂≅Δλ₂₃ . . . ≅Δλ_((n−1)n) (±10%) defined in the firstaspect is satisfied to obtain an almost incoherent state within apractical range. For this reason, even if the resist thickness or thedepth of the mark step difference slightly changes within the incoherentrange (e.g., 0.5 μm to 1.7 μm), the mark detection position obtained byphotoelectrically detecting a multi-wavelength interference light beamobtained from the ±1st-order diffracted light components generated fromthe mark rarely varies by the averaging effect of the multi-wavelengthlight beam obtained using beam having three or more wavelengths.

When a multi-wavelength light beam is obtained from coherent light beamshaving three or more wavelengths satisfying the conditions defined bythe first aspect, the influence of asymmetry caused by variations inmark step difference and resist thickness is almost eliminated, and goodposition detection precision can be always maintained.

As previously described, the amplitude reflectance of the grating markitself greatly varies depending not only on the mark depth and resistthickness but also the wavelengths of the illumination light beam(detection light beam). The detection light beam has a plurality ofwavelengths (broad band), the amplitude reflectances of the mark itselfare different in units of wavelength components, and position detectionresults are different accordingly. When the amplitude reflectances ofthe mark itself are assumed under various mark conditions, the positiondetection precision can be simulated.

The second aspect of the invention is based on the simulation resultsobtained as follows. Assume that a grating mark is to be irradiated withan illumination light beam having only a specific wavelength, and thatdiffracted light components generated by this grating mark are to bephotoelectrically detected. If the intensity (i.e., the amplitude of alevel change in signal during relative scanning between the illuminationlight beam and the grating mark) of a photoelectric signal becomesextremely low, the position detection precision is also degraded.According to the simulation results, in the second aspect, even underthe grating mark condition that use of an illumination light beam havinga single wavelength causes an extreme reduction in amplitude of aphotoelectric signal, an averaged mean of the position detection resultsfor each of the wavelengths using an illumination light beam havinganother wavelength is obtained to prevent extreme degradation of theposition detection precision.

Even under the conditions shown in FIG. 1 or 2, when a semiconductorlaser generates an illumination beam having a wavelength λ of 0.670 μmor 0.725 μm, the mark position detection error can be sufficientlyreduced. Judging from this fact, it is effect to use two-colorillumination beams having different wavelengths, such as an He—Ne laserand a semiconductor laser and give an attention (selection or weighting)to a mark position (position offset amount) detected upon beamirradiation having a wavelength which conducts a large amplitude in thedetected signal (AC component).

Alternatively, another method is also available in which only aninterference light beam of two 1st-order diffracted light componentspropagating in one specific direction is not detected, but aninterference light beam of the 0th- and 2nd-order diffracted lightcomponents propagating in another direction is photoelectricallydetected, and the mark position determined on the basis of thephotoelectric signal is taken into consideration. FIG. 4 showsgeneration of 0th-, ±1st-, and ±2nd-order diffracted light componentsunder the beam incident conditions that two irradiation beams ±L₁ havinga wavelength λ₁ and two irradiation beams ±L₂ having a wavelength λ₂ areincident on a diffraction grating mark MG, and interference fringeshaving a pitch Pif and the same intensity distribution on the gratingmark MG at the wavelengths λ₁ and λ₂ when the pitch Pmg of the gratingmark satisfies relation Pmg=2Pif.

In FIG. 4, an interference beam BM of the 1st-order diffractedcomponents ±D_(1n) propagating in a direction perpendicular to thesurface of the grating mark MG has wavelengths λ₁ and λ₂. The 0th-orderdiffracted component (normal reflected light component) derived from thebeams ±L₁ has an incident angle slightly different from that of thebeams ±L₂. For this reason, four beams ±D₀₁ and ±D₀₂ corresponding tothe beams ±L₁ and ±L₂ propagate in different directions. The firstsuffix in D₀₁ and D₀₂ indicates the diffraction order, and the secondsuffix indicates the wavelength (λ₁ or λ₂).

The 2nd-order diffracted light component −D₂₁ generated upon irradiationof the beam +L₁ propagates in a direction opposite to that of theoptical path of the beam +L₁ and interferes with the 0th-orderdiffracted light component +D₀₁ of the beam −L₁. Similarly, theremaining 2nd-order diffracted light components +D₂₁, −D₂₂, +D₂₂propagate in the same direction as that of the corresponding 0th-orderdiffracted light components −D₀₁, +D₀₂, and −D₀₂. The intensities of theinterference light beams between these 0th- and 2nd-order diffractedlight components change in accordance with relative changes between thegrating MG and the interference fringes as in the interference beam BMof the ±1st-order diffracted light components.

Assume only the wavelength λ₁. The 1st-order component (i.e., theinterference beam BM of the 1st-order diffracted light components ±D₁₁)is photoelectrically detected to obtain the mark position (or positionoffset). At the same time, two 2nd-order components (i.e., theinterference light beam of the 0th-order diffracted light component +D₀₁and the 2nd-order diffracted light component −D₂₁, and the interferencelight beam of the 0th-order diffracted light component −D₀₁ and the2nd-order diffracted light component +D₂₁) are photoelectricallydetected. A value obtained by averaging the mark positions respectivelyobtained using the signals having these two 2nd-order components isdefined as the position of the mark. The mark position detected usingthe 1st-order component or the mark position detected using the2nd-order components is selected in accordance with the magnitudebetween the average of the amplitude values of the 1st-order componentsignals and the average of the amplitudes of the 2nd-order componentsignals. Alternatively, a weighted mean is obtained.

As described above, the order of the diffracted light components usedfor mark detection is changed because the directions of the diffractedlight components generated by the grating MG are different depending onthe order. Even if the amplitude of the change in intensity of aninterference light beam of a given order component propagating in agiven direction becomes small to degrade the detection precision, theamplitude of the change in intensity of an interference light beam ofanother order propagating in another direction does not become small,thereby preventing degradation of the detection precision.

This can also be confirmed from the simulation results shown in FIGS. 5Aand 5B. FIGS. 5A and 5B are simulation graphs showing the relationshipsbetween the amplitudes of changes in signals (AC components) and theposition detection errors using the step difference T₂ of the grating MGin FIG. 2 as a parameter when the He—Ne laser generates an irradiationbeam having a wavelength of 0.633 μm. In these graphs, pitch Pmg=8 μm,duty=1:1, taper amount (ratio) ΔS=0.1%, and the thickness T₁ of theresist layer PR on the top surface is set to be 1.15 μm. FIG. 5A showsthe simulation for the interference beam BM of the 1st-order diffractedlight components (1st-order diffracted light components ±D₁₁), whileFIG. 5B shows the simulation for the 2nd-order (0th-order diffractedlight components ±D₀₁ and 2nd-order diffracted light components ±D₂₁)interference beams.

As can be understood from FIGS. 5A and 5B, the amplitude components ofthe signals obtained by photoelectrically detecting the interferencelight beams of the 1st- and 2nd-order diffracted light componentsgreatly change with a small change in the shape (step difference T₂) ofthe grating mark. For example, in FIG. 5A, when the grating stepdifference T₂ is 0.86 μm, the amplitude of the change in intensity ofthe 1st-order diffracted components becomes very small, and the positiondetection error is abruptly increased accordingly. However, in FIG. 5B,when the step difference T₂ is 0.86 μm, the change in intensity of the2nd-order interference light beam is relatively large, and degradationof the position detection error is small. Note that the amplitudes ofchanges in signals in FIGS. 5A and 5B are expressed as relative values,but the scales in FIGS. 5A and 5B are the same.

When an algorithm using both grating mark position detection using theinterference light beam of the 1st-order diffracted light components andgrating mark position detection using the interference light beam of the2nd-order diffracted light components and employing one of the results,a weighted mean of the detection positions (or positional offsets)obtained from the illumination light beam having a plurality ofwavelength components may be preferably obtained using the wavelengthdependence, as can be apparent from the simulation in FIG. 1.

The detection light has a plurality of wavelengths, and the pieces ofmark position information obtained in units of wavelength components areaveraged to perform higher-precision position detection than theconventional case. As shown in FIG. 1, according to the simulationresults, when the amplitude of a change (AC component) of a light amountsignal of a diffracted light beam (interference light beam) having agiven wavelength is small, a probability of degrading the positiondetection precision using the diffracted light beam having thiswavelength is high. In detecting diffracted light beams (interferencelight beams) of a plurality of wavelength components, the mark positionsdetected in units of wavelength components are weighted with smallweighting factors for small amplitudes of changes in signals, and largeweighting factors for large amplitudes of changes in signals. Theweighted components are then averaged. With this operation, thedetection result of the mark position using a diffracted light componenthaving a wavelength component having a high probability of a large erroris automatically weighted with a small weighting factor. Therefore, theprecision of the final mark position detection result can be maintainedto a certain degree.

In detecting a 2nd-order (an interference light beam of the 0th- and2nd-order diffracted light beams) signal, in order to individuallyobtain the mark positions using the signals photoelectrically detectedin units of wavelength components, the failure in detection of the markpositions caused by the canceling effect of the respective wavelengths(to be described later) at the time of reception of the diffracted lightbeams (interference light beams) can be prevented.

According to the present invention, the incident beams of the respectivewavelengths are irradiated while being sequentially switched one by onein grating mark detection. For this reason, a photoelectric element forphotoelectrically detecting an interference light beam of the 1st-orderdiffracted light components and a photoelectric element forphotoelectrically detecting an interference light beam of the 2nd-orderdiffracted light components need not be prepared for a plurality ofsets. In addition, a wavelength selection means for discriminating theinterference light beams in units of wavelength components can beomitted.

In order to solve the problem when the position of the grating patternis measured by the use of an illuminating light beam, the presentinvention premises that the position or the position offset amount of asubstrate on which a one-dimensional or two-dimensional diffractiongrating-like pattern is formed is detected by the use of a coherentilluminating beam (light of a single wavelength). Further, the presentinvention is constructed so as to be also applicable to a positiondetecting apparatus of any of the conventional homodyne type and theconventional heterodyne type by some improvement. So, as an example, theepitome of the invention of an apparatus (method) for projecting twoilluminating beams onto a grating pattern at symmetrical angles ofincidence to thereby detect the position of the grating mark willhereinafter be described with reference to FIGS. 49 and 52.

First, in the present invention, provision is made of beam applyingmeans (LS, TBO, AP, G1, G2) for applying two coherent illuminating beams(+LF, −LF) to a diffraction grating-like grating pattern (MG) formed ona substrate (W) of which the position is to be detected, at symmetricalangles of incidence. Provision is further made of a first photoelectricdetector (DT1) for receiving a first interference beam (BM) obtained bythe interference between two diffracted lights (±1st-order diffractedlights) travelling in a first direction (vertical direction) from thegrating pattern (MG), and second photoelectric detectors (DT2 a, DT2 b)for receiving second interference beams (BmO2, Bm2O) obtained by thedifference between two diffracted lights (0-order and 1st-orderdiffracted lights) travelling in a second direction differing from thefirst direction from the grating pattern (MG).

Further, by scanning means such as a stage (WST) for moving thesubstrate (W) or an optical system TBO for creating two beams whichprovides a predetermined frequency difference between the twoilluminating beams, the grating pattern (MG) is controlled so as to berelatively scanned by an interference fringe produced by the twoilluminating beams (+LF, −LF).

As shown in FIG. 52, provision is also made of weight coefficientcalculating means (amplitude ratio detection circuit 58) for calculatinga first weight coefficient (C11) conforming to the ratio between theamplitude value of a first photoelectric signal (Im1) outputted from thefirst photoelectric detector (DT1) during the relative scanning and thesubstantially ideal amplitude value of the first photoelectric signal,and calculating a second weight coefficient (C22) conforming to theratio between the amplitude value of second photoelectric signals (ImO2,Im2O) outputted from the second photoelectric detectors (DT2 a, DT2 b)and the substantially ideal amplitude of the second photoelectricsignals.

Provision is further made of position calculating means (detectioncircuit 56) for calculating first position information (ΔX11) forspecifying the position of the grating pattern (MG) on the basis of therelation between a level change in the first photoelectric signal and aposition which provides the reference of the relative scanning (or thereference in terms of time) and calculating second position information(ΔX22) for specifying the position of the grating pattern (MG) on thebasis of the relation between a change in the level of the secondphotoelectric signals and a position which provides the reference of therelative scanning (or the reference in terms of time), and weighted meancalculating means (60) for weight-averaging the first positioninformation and the second position by the first weight coefficient andthe second weight coefficient and determining the most apparentlycertain position (ΔX) of the grating pattern (MG).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the simulation results of wavelengthdependence of position detection errors when a conventional interferencemark position detection apparatus employing a single wavelength is used;

FIG. 2 is a partial sectional view showing the sectional shape of agrating mark used as a simulation model in FIG. 1;

FIG. 3 is a graph showing the simulation results of changes inreflectance of a substrate with a resist under multi-wavelengthconditions determined by the present invention;

FIG. 4 is a view showing the state of diffracted light components of therespective orders generated by the grating mark upon irradiation oflight beams having a plurality of wavelengths;

FIGS. 5A and 5B are graphs showing the simulations of the detectionerrors caused upon detection of a mark having the structure shown inFIG. 2 using the 1st-order diffracted light components and the detectionerrors caused upon detection of the mark using the 0th- and 2nd-orderdiffracted light components;

FIG. 6 is a view showing the arrangement of a position detectionapparatus according to the first embodiment of the present invention;

FIG. 7 is a perspective view showing the detailed structure of a rotaryradial grating plate in FIG. 6 and the generated states of diffractedlight components;

FIG. 8 is a graph showing the simulation results of changes inreflectance of a substrate with a resist when the combination ofwavelengths set in FIG. 3 is changed to another combination while themulti-wavelength conditions determined by the present invention are keptunchanged;

FIG. 9 is a block diagram showing the arrangement of a signal processingcircuit applied to the apparatus shown in FIG. 6;

FIGS. 10A and 10B are charts showing the waveforms of two photoelectricsignals to be processed in the signal processing circuit shown in FIG.9;

FIG. 11 is a partial sectional view illustrating the sectional structurebetween a grating mark having a small step difference formed on asemiconductor wafer and a resist layer covering the surface of the mark;

FIG. 12 is a view showing the arrangement of a position detectionapparatus according to the second embodiment of the present invention;

FIGS. 13A to 13D are views illustrating the position detection states ofa grating mark by the apparatus shown in FIG. 12 and the changes inphotoelectric signals;

FIG. 14 is a view showing the arrangement of a position detectionapparatus according to the third embodiment of the present invention;

FIG. 15 is a view showing the arrangement of a projection exposureapparatus to which the position detection apparatus of the presentinvention is applied according to the fourth embodiment of the presentinvention;

FIG. 16 is a view showing the arrangement of part of a TTL alignmentsystem in FIG. 15;

FIGS. 17A-17D are views showing an arrangement of a projection exposureapparatus to which the position detection apparatus of the presentinvention is applied according to the fifth embodiment of the presentinvention;

FIG. 18 is a view showing the arrangement of part of a positiondetection apparatus according to the sixth embodiment of the presentinvention;

FIG. 19 is a view showing the arrangement of a position detectionapparatus according to the seventh embodiment, in which spectraldetection systems for receiving interference beams are arranged in unitsof wavelength components;

FIG. 20 is a block diagram showing the arrangement of a signalprocessing circuit applied to the apparatus shown in FIG. 19;

FIG. 21 is a view showing the arrangement of a position detectionapparatus according to the eighth embodiment of the present invention;

FIGS. 22A to 22D are views showing changes in relative positionalrelationship between interference fringes and a grating, and changes inlevels of detection signals;

FIG. 23 is a view showing the arrangement of a position detectionapparatus according to the ninth embodiment of the present invention;

FIG. 24 is a view showing the arrangement of a position detectionapparatus according to the 10th embodiment of the present invention;

FIG. 25 is a block diagram showing a signal processing circuit appliedto the apparatus of the 10th embodiment;

FIGS. 26A to 26D are charts showing the waveforms of signals received inthe memory of the processing circuit shown in FIG. 25;

FIG. 27 is a block diagram showing a modification of the signalprocessing circuit applied to the apparatus shown in FIG. 24 as the 11thembodiment;

FIG. 28 is an enlarged view showing part of a TTL alignment of theapparatus shown in FIG. 15;

FIGS. 29A to 29D are charts showing the waveforms of photoelectricsignals of the respective wavelengths obtained from the interferencelight beams of the 0th- and 2nd-order diffracted light components from adiffraction grating;

FIGS. 30A to 30D are charts showing the waveforms of photoelectricsignals of the respective wavelengths obtained from the interferencelight beams of the 0th- and 2nd-order diffracted light components from adiffraction grating;

FIG. 31 is a view showing the arrangement of a position detectionapparatus according to the 12th embodiment of the present invention;

FIG. 32 is a block diagram showing the arrangement of a signalprocessing circuit applied to the apparatus shown in FIG. 31;

FIG. 33 is a sectional view showing part of an apparatus according tothe 13th embodiment of the present invention;

FIG. 34 is a view showing the arrangement of a position detectionapparatus according to the 14th embodiment of the present invention;

FIG. 35 is a view showing a modification of an illumination beamprojection scheme shown in each embodiment of the present invention;

FIG. 36 is a view showing the arrangement of beams on a Fouriertransform plane according to the illumination beam projection scheme inFIG. 35;

FIG. 37 is a view showing the arrangement of a position detectionapparatus according to the 15th embodiment of the present invention;

FIG. 38 is a block diagram showing a signal processing circuit appliedto the apparatus shown in FIG. 37;

FIGS. 39A to 39D are charts showing the waveforms of signals received ina memory in the processing circuit shown in FIG. 38;

FIG. 40 is a view showing the arrangement of a position detectionapparatus according to the 16th embodiment of the present invention;

FIG. 41 is a view showing the arrangement of a position detectionapparatus according to the 17th embodiment of the present invention;

FIG. 42 is a view showing the arrangement of a position detectionapparatus according to the 18th embodiment of the present invention;

FIG. 43 is a block diagram showing the arrangement of a signalprocessing circuit applied to the apparatus shown in FIG. 42;

FIG. 44 is a view for explaining a memory bank arrangement in a waveformmemory circuit unit in FIG. 43;

FIG. 45 is a sectional view showing part of an apparatus according tothe 19th embodiment of the present invention;

FIG. 46 is a view showing a position detection apparatus according tothe 20th embodiment of the present invention;

FIG. 47 schematically shows the state of diffracted light of each ordercreated from a grating mark when coherent illuminating beams areprojected from two symmetrical directions to the grating mark;

FIGS. 48A and 48B are graphs showing the result of the simulation of therelation between a position detection error using 1st-order diffractedlights from the grating mark and the step difference of the mark, andthe relation between a position detection error using 2nd-orderdiffracted lights from the grating mark and the step difference of themark;

FIG. 49 schematically shows the construction of a position detectingapparatus according to a twenty-first embodiment of the presentinvention;

FIG. 50 is a plan view showing an example of the construction of anoptical system TBO for creating two beams in the apparatus of FIG. 49;FIG.

FIG. 51 is a view of the construction of the optical system TBO forcreating two beams in FIG. 50 as it is seen from sideways thereof;

FIG. 52 is a block diagram showing the circuit construction of thesignal processing system of the apparatus shown in FIGS. 49 and 50;

FIGS. 53A to 53D are graphs showing the states of the waveforms ofrespective signals processed by the signal processing system of FIG. 52;

FIG. 54 schematically shows the construction of a position detectingapparatus according to a twenty-second embodiment of the presentinvention;

FIG. 55 is a block diagram showing the circuit construction of thesignal processing system of the apparatus shown in FIG. 54;

FIGS. 56A and 56B are graphs showing the states of the waveforms ofrespective signals processed by the signal processing system of FIG. 55;

FIGS. 57A and 57B are waveform graphs illustrating a case where eachsignal is processed by phase difference measurement when the apparatusof FIG. 54 is used in a homodyne system;

FIG. 58 shows the construction of a TTL alignment system in atwenty-third embodiment in which the position detecting apparatus of thepresent invention is applied to the various alignment systems of aprojection exposure apparatus;

FIGS. 59A-59D show the construction of a TTR alignment system in FIG.58; and

FIG. 60 is a plan view showing a modification of a correction plate PGPin FIGS. 59A and 59B.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The first embodiment of the present invention will be described withreference to FIGS. 6 and 7 and exemplifies a position detectionapparatus using a heterodyne scheme. Referring to FIG. 6, three laserlight sources LS₁, LS₂, and LS₃ generate laser beams LB₁, LB₂, and LB₃having different wavelengths λ₁, λ₂, and λ₃, respectively. For example,the laser light source LS₁ is an He—Ne laser light source of λ₁=0.633μm, the light source LS₂ is a semiconductor laser light source ofλ₂=0.690 μm, and the light source LS₃ is a semiconductor laser lightsource of λ₃=0.760 μm. The wavelength relationship is given as λ₁<λ₂<λ₃.

All these laser light sources are commercially available laser lightsources. In consideration of heat dissipation of laser tubes andcompactness, if all three light sources are to be constituted bysemiconductor laser light sources, a beam from a semiconductor laserlight source having an oscillation wavelength of 1.20 μm is incident ona second-harmonic generator (SHG) to form a beam having a wavelengthλ₁=0.600 μm. This beam may be used in place of the beam LB₁ from theHe—Ne laser light source LS₁ in FIG. 6. At this time, the light sourceLS₂ in FIG. 6 is a semiconductor laser light source having a wavelengthλ₂=0.640 μm, and the light source LS₃ is a semiconductor laser lightsource having a wavelength λ₃=0.690 μm.

In this case, the following relations are established:

Δλ₁₂=1/λ₁−1/λ₂=0.1042

 Δλ₂₃=1/λ₂−1/λ₃=0.1132

so that Δλ₂₃/Δλ₁₂=1.087 (an error of 8.7%) can be established, therebysatisfying the conditions defined by the present invention within therange of ±10%.

The state of temporal coherence at this time is shown as the simulationresults in FIG. 8 as in the previous drawing. As can be understood fromthe graph of FIG. 8, the three wavelengths are shifted to theshort-wavelength side as a whole as compared with the case in FIG. 3.The incoherent region is shifted to a range (0.7 μm to 2.0 μm) in whichthe resist thickness (or mark step difference) increases. However, inthis range (0.7 μm to 2.0 μm), the variation in reflectance is verysmall. As in FIG. 3, an incoherent arrangement can be properly obtained.

Referring back to FIG. 6, the three beams LB₁, LB₂, and LB₃ aresynthesized into one beam LB₀ through a mirror MR and dichroic mirrorsDCM₄ and DCM₅. The beam LB₀ is reflected by the mirror MR and incidenton a rotary radial grating plate RRG. This rotary grating plate RRG isrotated about a rotation axis C₀ at a high constant angular velocity inone direction. This grating plate RRG serves as a frequency modulator(frequency shifter) for changing the frequencies of the diffracted lightbeams of the respective orders in accordance with the angular velocity.

FIG. 7 is an enlarged perspective view of the rotary radial gratingplate RRG. The rotation axis C₀ is set parallel to the Z-axis of theX-Y-Z coordinate system. A transmission phase diffraction grating RG isformed on the circular grating plate RRG in the angular range of 360°.When the beam LB_(o) is vertically incident on the grating RG of thegrating plate RRG, various diffracted light components in addition to a0th-order diffracted light component D₀ are generated. In thisembodiment, the heterodyne scheme is realized using the ±1st-orderdiffracted light components. For this reason, only the ±1st-orderdiffracted light components from the grating plate RRG are illustratedin FIGS. 6 and 7.

First-order diffracted beams ±D₁₁ generated from the beam LB₁ having thewavelength λ₁, 1st-order diffracted beams ±D₁₂ generated from the beamLB₂ having the wavelength λ₂, and 1st-order diffracted beams ±D₁₃generated from the beam LB₃ having the wavelength λ₃ are generated fromthe grating RG of the rotary grating plate RRG. A diffraction angle θnof the 1st-order diffracted beam for each wavelength is represented asfollows:

sin θ_(n)=λ_(n) /Prg

where n is the number of wavelengths, and Prg is the pitch of thegrating RG.

Each 1st-order diffracted beam is subjected to a predetermined frequencyshift Δf regardless of the wavelengths. If a velocity at which thegrating RG of the grating plate RRG crosses the beam LB₀ is defined asV, Δf=V/Prg is obtained. A +1st-order diffracted beam has a frequencyhigher than the frequency of the 0th-order diffracted light component byΔf, while a −1st-order diffracted beam has a frequency lower than thefrequency of the 0th-order diffracted light component by Δf. Therefore,the rotary radial grating plate RRG serves as a frequency shifter.

As shown in FIG. 6, incident beams ±LF consisting of the 1st-orderdiffracted beams ±D_(1n) (n=1, 2, 3) having the three wavelengthcomponents and the 0th-order diffracted light component D₀ are convertedby a collimator lens 10 such that their principal rays are parallel toeach other. These beams reach a beam selection member 12. The beamselection member 12 serves as a spatial filter located on the-so-calledFourier transform plane. The beam selection member 12 shields the0th-order diffracted light component D₀ and passes the incident beams±LF derived from the 1st-order diffracted light components ±D_(1n).

The incident beams ±LF then reach a beam splitter (half mirror) 20through adjustment optical systems 14, 16, and 18 constituted byplane-parallel glass members whose inclination amounts are variable. Theadjustment optical system 14 has a function of shifting the incidentbeams ±LF with respect to the optical axis of the lens 10 while thedistance between the incident beams +LF and −LF in the Fourier space iskept unchanged. The adjustment optical systems 16 and 18 have functionsof individually adjusting the incident beams +LF and −LF with respect tothe optical axis. The incident beams ±LF are split into two beams by thebeam splitter 20. One pair of the two beams is incident on an objectivelens 22 serving as an irradiation optical system, while only the1st-order diffracted beams having a specific wavelength in the incidentbeams ±LF, i.e., only the 1st-order diffracted beams ±D₁₂ having thewavelength λ₂ of the other beam are selected and incident on a condenserlens (Fourier transform lens) 26 through a filter 24.

The incident beams ±LF incident on the objective lens are collimatedinto parallel beams which are then simultaneously irradiated on thegrating MG on a wafer W at different angles. Interference fringes formedby the interference of the incident beams ±D₁₁ having the wavelength Δ₁,interference fringes formed by the interference of the incident beams±D₁₂ having the wavelength λ₂, and interference fringes formed by theinterference of the incident beams ±D₁₃ having the wavelength λ₃ aresuperposed on each other and appear on the grating MG at the same pitchand the same phase. The interference fringes are observed as if they aremoving on the grating MG at a constant speed in one direction because ofthe frequency difference 2Δf between the incident beams +LF and −LF.This moving speed is proportional to the velocity V of the grating RG ofthe rotary radial grating plate RRG.

As can be apparent from FIG. 6, the surface (grating MG) of the wafer Wand the radial grating plate RRG are located conjugate to each other(imaging relationship) by a composite optical system of the collimatorlens 10 and the objective lens 22. For this reason, the images obtainedby the ±1st-order diffracted light components on the grating RG of theradial grating plate RRG are formed in a size corresponding to themagnification factor of the composite optical system on the grating MGof the wafer W. The diffraction images (interference intensitydistribution) projected on the wafer W are formed ½ of the pitch of thegrating RG because the 0th-order diffracted light component D₀ isshielded. The pitch Pif of the interference fringes on the wafer W is ½of the pitch Pmg of the grating MG.

With the above arrangement, incident angles Φ_(n) (n is the number ofwavelengths; 1, 2, 3) of the incident beams ±D₁₁, ±D₁₂, and ±D₁₃ of therespective wavelength components irradiated onto the grating mark MG ofthe wafer W upon emergence from the objective lens 22 are defined asfollows:

sin Φ_(n)=±λ_(n) /Pmg

When this relation is satisfied, the 1st-order diffracted lightcomponents are generated vertically from the grating MG upon irradiationof the incident beams ±LF. More specifically, an interference beam BM isobtained by interfering the 1st-order diffracted light componentsvertically generated upon irradiation of the incident beam +LF and the1st-order diffracted light components vertically generated uponirradiation of the incident beam −LF. This interference beam BM is abeat light beam intensity-modulated with the frequency 2Δf. In thismanner, to generate the ±1st-order diffracted light beams (interferencebeam BM) in the same direction, distances DL_(n) of the incident beams±LF of the respective wavelengths on the Fourier transform plane fromthe optical axis are defined as follows from the different viewpoint:

DL _(n) =F ₀·sin Φ_(n) =±F ₀·λ_(n) /Pmg (n=1, 2, 3)

where F₀ is the focal length of the objective lens 22 and Φn is theincident angle of each wavelength component of each incident beam. Thedistance DL_(n) for each wavelength can be adjusted by appropriatelysetting the pitch of the grating RG of the rotary radial grating plateRRG and the focal length of the collimator lens 10.

The interference fringes formed on the wafer W are imaged as adiffraction image of the grating RG of the radial grating plate RRG. Inprinciple, if the pitch of the interference fringes obtained by one ofthe wavelength components having the three wavelengths λ₁, λ₂, and λ₃ isan integer multiple of the pitch of the grating mark MG, the pitches ofthe interference fringes for the remaining wavelength components havethe same relationship. The interference fringes obtained in units ofwavelength components must perfectly match and be free from phaseoffsets and position offsets.

In practice, however, the interference fringes obtained in units ofwavelength components are subjected to position offsets, phase offsets,and pitch offsets in accordance with the degree of chromatic aberrationof the optical systems such as the objective lens 22 and the collimatorlens 10.

To correct these offsets, the adjustment optical systems 14, 16, and 18in FIG. 6 are used. These optical system 14, 16, and 18 are constitutedby plane-parallel glass members. When a material having a large colordispersion is used as the material of these optical systems, theposition and phase offsets of the interference fringes formed on thewafer in units of wavelength components can be slightly changed.Alternatively, a plane-parallel glass member having a small colordispersion may be combined with a plane-parallel glass member having alarge color dispersion to constitute the adjustment optical systems 14,16, and 18. In this case, the inclination of the plane-parallel glassmember having the large color dispersion is adjusted to correct therelationship between the interference fringes for each wavelengthcomponent. An error in the overall inclination of the incident beams ±LFon the wafer, which is caused by the above inclination adjustment, canbe corrected by inclination adjustment by the plane-parallel glassmember having the small color dispersion.

The interference beam BM vertically generated by the interferencefringes from the illuminated grating MG reaches the spatial filter 28through the objective lens 22 and the beam splitter 20. This spatialfilter 28 is located at or near the Fourier transform plane associatedwith the objective lens 22. In this embodiment, the spatial filter 28has an aperture for passing only the interference beam MB (±1st-orderdiffracted light components). The interference beam MB passing throughthe spatial filter 28 is converted into a parallel beam by a lens system(inverse Fourier transform lens) 30. The parallel beam is reflected by amirror 32 and received by a photoelectric element 36A.

This photoelectric element 36A simultaneously receives the interferencebeam BM including three wavelengths λ₁, λ₂, and λ₃. This interferencebeam BM is intensity-modulated with the beat frequency 2Δf. For thisreason, a photoelectric signal I_(m1) from the photoelectric element 36Ahas an AC waveform whose level sinusoidally changes at the samefrequency as the beat frequency 2Δf while the interference beam BM fromthe grating mark GM is present.

On the other hand, the 1st-order beams ±D₁₂ selected by the wavelengthselection filter 24 and incident on the condenser lens 26 are superposedon each other and irradiated on a transmission reference grating SG. Inthis case, the reference grating SG is located conjugate to the rotaryradial grating plate RRG with respect to the composite optical system ofthe collimator lens 10 and the condenser lens 26. For this reason,one-dimensional interference fringes are formed on the reference gratingSG by the two-beam interference of the 1st-order beams ±D₁₂. Theseinterference fringes move at a speed corresponding to the beat frequency2Δf.

When the pitch of the reference grating SG and the pitch of thecorresponding interference fringes are appropriately set, the ±1st-orderdiffracted light components generated from the reference grating SGpropagate as an interference beam B_(ms) in the same direction, passesthrough a spatial filter 38, and is received by a photoelectric element40. A photoelectric signal I_(ms) from this photoelectric element 40 hasa waveform whose level sinusoidally changes at the same frequency as thebeat frequency 2Δf. This signal I_(ms) serves as a reference signal inthe heterodyne scheme.

In the above arrangement, the reference grating SG is formed such that achromium layer is deposited on a glass plate and etched to alternatelyform transparent and light-shielding lines. For this reason, an almostideal grating, i.e., a grating having symmetrical amplitudetransmittances, almost free from at least asymmetry as in the gratingmark MG on the wafer W and the problem posed by the resist layer isformed. The pair of incident beams irradiated on the reference gratingSG may be incident beams having one of the wavelengths λ₁, λ₂, and λ₃ soas to obtain a sufficiently high precision. On the contrary, it isavailable that all the three 1st-order diffracted beams ±D₁₁, ±D₁₂, and±D₁₃ included in the incident beams ±LF may be simultaneously incidenton the reference grating SG, thereby forming multi-color interferencefringes as in the grating mark MG on the wafer.

As described above, the multi-color interference fringes are formed onthe reference grating SG, and the interference beam B_(ms) generatedfrom this interference grating SG is separated in units of wavelengthsand photoelectrically detected. A reference signal corresponding to thewavelength λ₁, a reference signal corresponding to the wavelength λ₂,and a reference signal corresponding to the wavelength λ₃ can beindividually obtained, so that position measurement of the grating markMG can be performed in units of wavelengths. Even if the interferencefringes formed on the wafer W by three wavelength components are offset(phase offset) by a predetermined amount, the offset amount can bemeasured in advance. This technique will be described in detail later.

The wafer W shown in FIG. 6 is placed on a wafer stage WSTtwo-dimensionally moved within a plane (X-Y plane) perpendicular to theoptical axis of the objective lens 22. The two-dimensional movement onthe stage WST is performed by a drive source 42 including a drive motor.This driving may be based on a scheme for rotating a feed screw by amotor or a scheme for directly moving the stage itself by a linearmotor. In addition, the coordinate position of the stage WST issequentially measured by a laser interferometer 44. The measurementvalues of the laser interferometer 44 are used for feedback control forthe drive source 42.

A fiducial mark plate FG is formed on part of the wafer stage WST. Areflection intensity grating (the pitch is equal to that of the gratingMG on the wafer) having a line-and-space pattern with a chromium layeron the surface of quartz glass is formed on the mark plate FG. For thisreason, unlike the phase grating such as the grating mark MG formed bycorrugations on the wafer W, the intensity grating is characterized inthat no asymmetry is present and the diffraction efficiency does notdepend on the wavelength of the illumination light beam (detection lightbeam), i.e., in that the amplitude reflectance does not have asymmetry.In addition, the reflectance of the chromium layer rarely changes in thewavelength band (generally 0.5 μm to 0.8 μm) of the position detectionillumination light beam.

In the arrangement shown in FIG. 6, semiconductor lasers are used as thelight sources. In this case, it is preferable that an astigmatismremoval shaping optical system (i.e., a plurality of inclinedplane-parallel glass members) be arranged between each semiconductorlaser (LS₂ and LS₃) and each of the dichroic mirrors DCM₄ and DCM₅, andthe diameters of the beam components of one synthesized beam LB₀ beequal to each other in units of wavelength components. In addition, itis preferable that a beam shaping optical system for aligning thediameters of the wavelength components of the synthesized beam LBO bearranged.

For the sake of descriptive simplicity, the rotary radial grating plateRRG is used as the frequency shifter in FIG. 6. However, twoacousto-optic modulators (AOMs) may be used, or a first Zeeman laserlight source for oscillating a laser beam having a center frequency λ₁and a second Zeeman laser source for oscillating a laser beam having acenter frequency λ₂ may be used as light sources. In use of a Zeemanlaser, it generally oscillates two laser beams whose polarizationdirections are complementary, and a frequency difference of severalhundreds kHz to several MHz is given between these two laser beams. Thebeat frequency of an interference beam to be photoelectrically detectedis increased to a degree corresponding to the frequency difference. Thephotoelectric elements 36A and 40 must be constituted by PIN photodiodesor photomultipliers having a high response speed.

Various dichroic mirrors shown in FIG. 6 may be replaced with dispersionelements such as prisms. In this case, one prism has the same functionas the set of two dichroic mirrors DCM₄ and DCM₅.

An arrangement of a position detection/control circuit suitable for theapparatus shown in FIG. 6 will be described with reference to FIG. 9. Inthe heterodyne scheme in FIG. 6, while the interference beam BM isgenerated from the grating mark MG on the wafer W and the fiducial markplate FG, the signals I_(m1) and I_(ms) from the photoelectric elements36A and 40 have sinusoidal AC waveforms shown in FIGS. 10A and 10B.

FIG. 10B shows a time change in intensity of the signal I_(ms) servingas a reference signal, while FIG. 10A shows a time change in intensityof the signal I_(m1) upon reception of the interference beam BM from thegrating mark MG on the wafer W. Assuming the phase of the signal I_(ms)as a reference, the phase of the signal I_(m1) is offset from the signalI_(ms) by −Δψ₁. Let E₁ be the amplitude (peak-to-peak value of the ACcomponent) of the signal I_(m1).

In the circuit block shown in FIG. 9, the signals I_(m1) and I_(ms) areinput to an analog-to-digital conversion (A/D converter) circuit unit50. The instantaneous intensity levels of the input signals areconverted into digital values in response to a clock signal (pulse)C_(ps) from a sampling clock generator 52. The frequency of the clocksignal C_(ps) is set much higher than the beat frequency of the signalsI_(m1) and I_(ms). The clock signal C_(ps) is also sent to a waveformmemory circuit or circuit unit 54 and is used to update the memoryaddress in storing the digital values (data) from the A/D converter 50.

In the waveform memory circuit unit 54, the two waveform data shown inFIGS. 10A and 10B are digitally sampled for the predetermined periods(e.g., 10 periods) of the signals I_(m1) and I_(ms). At this time, sincethe two signals I_(m1) and I_(ms) are simultaneously sampled by thecommon clock signal C_(ps), the waveform data in the waveform memorycircuit unit 54 are not offset along the time axis. Note that when therotary radial grating plate RRG is used, the clock signal C_(ps) has afrequency of about ten and several kHz because the several kHz are theupper limit of the beat frequency. As in reference (E) (Japanese PatentApplication Laid-open No. 6-82215), when the frequency shifterconstituted by the two AOMs arranged in tandem with each other is used,the beat frequency is determined by twice the difference between thefrequencies of the high-frequency modulation signals applied to therespective AOMs and can be relatively freely determined.

The waveform data in the memory circuit unit 54 are loaded in a phasedifference Δψ and position offset amount ΔX calculation circuit orcircuit unit 56, so that the phase difference Δψ₁ shown in FIGS. 10A and10B is calculated by a digital arithmetic operation (Fourierintegration). If the pitch Pmg of the grating mark MG of the wafer W andthe pitch Pif of the interference fringes formed thereon satisfyrelation Pmg=2Pif, one period of each waveform in FIGS. 10A and 10Bcorresponds to Pmg/2.

Phase measurement is generally performed within the range of ±180°. Thecalculation circuit unit 56 converts the calculated phase difference Δψ₁into a position offset amount ΔX within the range of ±Pmg/4 inaccordance with the conversion formula ΔX=Pmg·Δψ₁/4π. This offset amountΔX represents an offset of the grating mark MG with respect to thereference grating SG within the range of ±Pmg/4. Assuming that theresolution of the phase difference measurement is given as about 0.2°,the resolution of the offset amount is about (0.2/180)Pmg/4. If thepitch Pmg is set to be 4 μm, a practical range of 0.002 μm (2 nm) isobtained.

The resultant offset amount ΔX is an offset of the grating mark MG withrespect to the reference grating SG in the pitch direction, and theoffset amount data is sent to a position controller (display controller)62 and to a servo control circuit or circuit unit 64 if the wafer W isaligned (positioned) in real time.

This servo control circuit unit 64 has two functions. One function is toperform feedback control for the drive source 42 until the offset amountΔX reaches a predetermined value (direct servo mode). To perform thisfunction, the A/D converter 50, the memory circuit unit 54, and thecalculation circuit unit 56 are repeatedly operated to calculate theoffset amount ΔX every very short period of time (e.g, several msec.)

The other function of the servo control circuit unit 64 is a function ofmoving the wafer stage WST on the basis of a measurement value from thelaser interferometer 44 (interferometer servo mode). This function isused to position the grating of the fiducial mark plate FG on the stageWST or the grating mark MG on the wafer W immediately below theobjective lens 22, or position an arbitrary point on the wafer Wimmediately below the objective lens 22 with reference to the detectedposition of the grating mark MG.

In the interferometer servo mode, target position information of thewafer stage WST is output from the position controller 62 to the servocontrol circuit unit 64. The control circuit unit 64 performs feedbackcontrol of the drive source 42 such that a difference between the targetposition and the current position of the stage WST which is read by thelaser interferometer 44 falls within a predetermined allowable range(e.g., ±0.4 μm).

When the direct servo mode is set following the interferometer servomode, the servo enable range of the direct servo mode is ±Pmg/4 withrespect to the pitch Pmg of the grating mark MG. If the detection valueis larger than the upper limit of this range, positioning is performedwhile an offset corresponding to ½ a pitch of the grating mark MG isleft uncorrected. The positioning allowable range of the stage WST inthe interferometer servo mode need not be steadily set to be ±0.04 μm,but may be switched to the allowable range of ±[(Pmg/4)−α] when thegrating mark MG (or the fiducial mark plate FG) is to be detected.

For example, if the allowable range is set to be about ±0.5 μm at thepitch Pmg of 4 μm, the positioning servo operation can be performed withless strict precision than that for the normal allowable range (±0.04μm), thereby shortening the tracing time. When the difference fallswithin the less strict allowable range (±0.05 μm), the direct servo modeis set to perform high-precision positioning (alignment) at a higherspeed.

In addition to the servo mode switching instruction, the positioncontroller 62 also has a function of displaying the coordinate positionof the grating mark MG and the calculated offset amount ΔX.

The position detection apparatus and its method according to the firstembodiment of the present invention have been described above. A gratingmark formed on a semiconductor wafer for alignment duringphotolithography process is formed as a relatively low step mark havinga relatively small step difference when the heterodyne (or homodyne)interference mark detection method is employed.

In the interference mark detection method, a mark itself is constitutedas a pattern of 10 to 20 lines and spaces. A diffracted light componentgenerated by the entire line-and-space pattern is photoelectricallydetected. Even if a grating mark has a small step difference, markposition detection can be performed on the basis of a sufficiently largedetection light amount.

FIG. 11 is an enlarged view showing part of the sectional structure of agrating mark MG′ having a small step difference in the pitch direction.The step difference of the grating mark MG′ is a step difference T₂between a mark surface Som and the bottom surface of the grating groove.Note that the model used in the simulation in FIG. 1 is the grating markMG having the sectional structure shown in FIG. 2. The assumed stepdifference T₂ was 0.7 μm. On the other hand, the mark having a, smallstep difference has a step difference T₂ of 0.5 μm or less in FIG. 11.This does not almost cause asymmetry during wafer processing. For thisreason, the variation in amplitude reflectance which is caused by atleast asymmetry can be minimized, thereby contributing to theimprovement of the precision in mark position detection.

An average thickness (target coating thickness) To (or T₁) of the resistlayer PR in FIG. 11 generally falls within the range of 0.7 μm to 1.2μm. For this reason, the change range of T₁ to T₃ of the substantialresist thicknesses in use of the grating mark MG′ having a small stepdifference satisfies T₃≅T₂+(T₁−ΔT)=T₁+0.72T₂, provided that a resistlayer surface Sor in the groove portion of the grating is recessed byΔT=0.3T₂. Therefore, the step difference T₂ of the grating mark MG′ isset to be 0.5 μm, and the change range of T₁ to T₃ falls within therange of 0.7 μm to 1.55 μm.

As can be apparent from the simulation results in FIG. 8, this rangeperfectly matches the incoherent range of the multi-wavelengthillumination light beams (±LF) obtained from the three wavelengths λ₁,λ₂ and λ₃.

In this embodiment, even if the grating mark has a small step differenceto almost eliminate asymmetry in the grating mark structure, theinfluence (variation in amplitude reflectance) of the resist layer whichposes the subsequent problem can also be reduced.

FIG. 12 shows the arrangement of a position detection apparatusaccording to the second embodiment of the present invention. In thiscase, a relative position offset amount between two diffraction gratingmarks RG and MG in the pitch direction (X direction) is measured by thehomodyne scheme. Beams LB₁, LB₂, and LB₃ as the illumination light beamsare emitted from laser light sources LS₁, LS₂, and LS₃ (FIG. 6) havingdifferent wavelengths λ₁, λ₂, and λ₃, synthesized coaxially as parallelbeams, and vertically irradiated on the grating mark RG through a mirrorMR1.

A plurality of diffracted light components are generated uponirradiation of the beams LB₁, LB₂, and LB₃ (parallel light beams) fromthe grating mark RG. When the grating mark RG is a one-dimensionaltransmission grating having a duty of 1:1, and its pitch direction isthe horizontal direction on the drawing surface in FIG. 12, thesediffracted light components (diffracted beams) are deflected on thedrawing surface of FIG. 12 at a predetermined diffraction angle.

As the diffracted beams in FIG. 12, 1st-order diffracted beams +D₁₁ and−D₁₁ generated from the beam LB₁ having the wavelength λ₁, 1st-orderdiffracted beams +D₁₂ and −D₁₂ generated from the beam LB₂ having thewavelength λ₂, 1st-order diffracted beams +D₁₃ and −D₁₃, and a 0th-orderdiffracted beam D₀ are illustrated. Diffracted light beams of higherorder are also generated from the beams LB₁, LB₂, and LB₃, but only the1st-order diffracted beams are illustrated for the sake of descriptivesimplicity.

Each diffracted beam is incident on an imaging optical system (i.e., aprojection exposure optical system and a mark detection alignmentoptical system) divided into a front-group lens system G1 and arear-group lens system G2. The grating mark RG is located at theposition of a front focal length f1 a of the front-group lens system G1.When the position of a rear focal length f1 b of the front-group lenssystem G1 almost coincides with the position of a front focal length f2b of the rear-group lens system G2 to form a Fourier transform plane(pupil plane) EP, the 1st-order diffracted beams cross (imaging) at theposition of a rear focal length f2 a of the rear-group lens system G2.Assume that the chromatic aberration of the imaging optical systemconsisting of the lens systems G1 and G2 is corrected for the threewavelengths λ₁, λ₂, and λ₃. As shown in FIG. 12, a small mirror MR2 isfixed at the center of the Fourier transform plane EP. The 0th-orderdiffracted beam D₀ from the grating RG is shielded by this mirror MR2and is prevented from being incident on the rear-group lens system G2.When the respective 1st-order diffracted beams emerge from the gratingmark RG, they are parallel beams as in the beams LB₁, LB₂, and LB₃ butare converged as beam waists at the position-of the Fourier transformplane EP by the behavior of the front-group lens system G1.

A diffraction angle θ₁ (i.e., an angle with respect to the 0th-orderdiffracted beam D₀) of the 1st-order diffracted beams ±D₁₁ generated bythe beam LB₁ having the wavelength λ₁, a diffraction angle θ₂ of the1st-order diffracted beams ±D₁₂ generated by the beam. LB₂ having thewavelength λ₂, and a diffraction angle θ₃ of the 1st-order diffractedbeams ±D₁₃ generated by the beam LB₃ having the wavelength λ₃ aredefined as follows:

sin θ₁=λ₁ /Prg  (1)

sin θ₂=λ₂ /Prg  (2)

sin θ₃=λ₃ /Prg  (3)

If λ₁<λ₂<λ₃, then θ₁<θ₂<θ₃. As shown in FIG. 12, on the Fouriertransform plane EP, the 1st-order diffracted beams ±D₁₁ pass inside the1st-order diffracted beams ±D₁₂ (on the side of 0th-order diffractedbeam), while the 1st-order diffracted beams ±D₁₂ pass inside the side of1st-order diffracted beams ±D₁₃ (on the 1st-order diffracted beams±D₁₁).

The respective 1st-order diffracted beams are superposed as parallelbeams on the reflection grating us mark MG formed by corrugations on awafer W through the rear-group lens system G2. In this case, the pitchdirection of the grating mark MG coincides with the X direction.One-dimensional interference fringes having the wavelength λ₁ (the pitchdirection is the X direction) are generated by two-beam interference ofthe 1st-order diffracted beams ±D₁₁, one-dimensional interferencefringes having the wavelength λ₂ (the pitch is X direction) are formedby two-beam interference of the 1st-order diffracted beams ±D₁₂, andone-dimensional interference fringes having the wavelength λ₃ (the pitchdirection is the X direction) are formed by two-beam interference of the1st-order diffracted beams ±D₁₃.

At this time, since the light beam having the wavelength λ₁, the lightbeam having the wavelength λ₂, and the light beam having the wavelengthλ₃ are different wavelengths each other, the 1st-order diffracted beams±D₁₁, ±D₁₂, and ±D₁₃ are not interfered with each other. It is importantthat the interference fringes having the wavelengths λ₁, λ₂, and λ₃generated by the 1st-order diffracted beams ±D₁₁, ±D₁₂, and ±D₁₃ appearas one set of interference fringes having the same intensitydistribution pitch.

An intensity distribution pitch Pif of the interference fringes isdetermined by a pitch Prg of the grating mark RG and a magnificationfactor M of the imaging optical system (G1 and G2) and is defined asPif=M·Prg/2. For example, when the pitch Prg is set to be 4 μm, and themagnification factor M is ¼ (the pattern size of the grating RG isreduced into ¼ on the grating mark MG side), the pitch Pif of theinterference fringes becomes 0.5 μm. If the pitch Pmg of the grating MGto be measured is given as Pmg=2Pif, i.e., Pmg=M·Prg, then rediffractedlight components of the respective wavelengths are generated from thegrating mark MG, by the each 1st-order diffracted beams ±D_(1n) as theincident light beams.

For example, one rediffracted light component generated by the gratingmark MG upon irradiation of the 1st-order diffracted beam +D₁₁ as theincident beam is the −1st-order diffracted light component (wavelengthλ₁) vertically propagating from the grating mark MG. One rediffractedlight component generated from the grating mark MG upon irradiation ofthe 1st-order diffracted beam −D₁₁ as the incident beam is the+1st-order diffracted light component (wavelength λ₁) verticallypropagating from the grating mark MG. The +1st-order diffracted lightcomponents of the wavelength λ₁ vertically propagating from the gratingmark MG have an interference intensity corresponding to the mutual phasestates, serve as an interference beam BM, and reach a mirror MR2.

Similarly, rediffracted light components are generated from the gratingmark MG upon irradiation of the 1st-order diffracted beams ±D₁₂ and ±D₁₃as the incident beams. The −1st-order diffracted light component of thewavelength λ₂ (wavelength λ₃) generated from the grating mark MG uponirradiation of the 1st-order diffracted beam +D₁₂ (+D₁₃) propagates in adirection perpendicular to the grating mark. The +1st-order diffractedlight component of the wavelength λ₂ (wavelength λ₃) generated from thegrating mark MG upon irradiation of the 1st-order diffracted beam −D₁₂(−D₁₃) propagates in a direction perpendicular to the grating MG. The+1st-order diffracted light components of the wavelengths λ₂ and λ₃propagating in the direction perpendicular to the grating mark MG alsohave an interference intensity corresponding to the mutual phase states,serve as the interference beam BM, and reach the mirror MR2.

Therefor the interference beam BM coaxially includes an interferencebeam B_(m1) having the wavelength λ₁, an interference beam B_(m2) havingthe wavelength λ₂, and an interference beam B_(m3) having the wavelengthλ₃.

The interference beam BM is reflected by the mirror MR2 and reaches aphotoelectric element DT through a lens system G3 constituting aphotoelectric detection system. The interference beams B_(m1), B_(m2),and B_(m3) of the respective wavelengths in the interference beam BM aresimultaneously received by the photoelectric element DT. Thephotoelectric element DT outputs a photoelectric signal I_(m1) havingthe level corresponding to the intensity of the interference beam BM toa circuit unit CU₃.

The circuit unit CU₃ is arranged as in the signal processing circuitshown in FIG. 9. However, since the homodyne scheme is employed, thesecond grating mark MG, i.e., a stage WST on which the wafer W is placed;Ls slightly moved in the pitch direction with respect to theinterference fringes in mark position detection. A change in level(waveform) of the signal I_(m1) output from the photoelectric element DTin mark position detection is changed and measured in correspondencewith the movement position of the stage. Note that the incident beams±D_(1n) may be moved without changing the crossing angle of the pair ofincident beams ±D_(1n) so as to move the interference fringes along thepitch direction.

The signal processing circuit of this embodiment can be realized bychanging the circuit arrangement in FIG. 9 as follows. A positionmeasurement pulse signal output from an interferometer shown in FIG. 6is supplied to an A/D converter 50 and a waveform memory circuit unit 54in place of a sampling pulse C_(ps) in the circuit block shown in FIG.9. The signal I_(m1) from the photoelectric element DT iswaveform-sampled with the pulse signal (e.g., one pulse for 0.02-μmmovement of the stage WST) from the interferometer 44. Note that areference signal I_(ms) need not be processed, unlike in the processingcircuit in FIG. 9 because the second embodiment employs the homodynescheme.

The circuit unit CU₃ in FIG. 12 has the same arrangement as that of theA/D converter 50, a waveform memory circuit 54, and a calculationcircuit 56 in FIG. 9. However, a method of calculating a position offsetamount ΔX in the calculation circuit 56 is different from that in theheterodyne scheme. That is, in the homodyne scheme, the sinusoidalsampling waveform stored in the waveform memory circuit 54 is not afunction of time, but a function of position. The waveform itselfrepresents the relative positional relationship in the grating mark MG.

Since the homodyne scheme is employed in this embodiment, the intensityof the interference beam BM changes in accordance with a change inrelative position between the grating marks RG and MG in the Xdirection. If the grating marks RG and MG are kept stopped in a givenstate, the levels of the signals I_(m1) continuously have predeterminedvalues. The grating mark MG and the interference fringes generated bythe grating mark RG on the grating mark MG are scanned relative to eachother by a predetermined amount (the distance of the pitch Pif or moreof the interference fringes) in the X direction, and the peak value andthe bottom value during the sinusoidal change in level of the signalI_(m1) generated during scanning are sampled by the circuit unit CU₃. Anamplitude value is specified from the difference between the peak andbottom values, and then the mark position offset amount ΔX iscalculated.

A change in the signal I_(m1) with a change in positional relationshipbetween the interference fringes and the grating mark MG will bedescribed with reference to FIGS. 13A to 13D. In FIGS. 13A, 13B, and13C, the interference fringes having the pitch Pif have a goodsinusoidal intensity distribution due to two-beam interference and areset to be Pmg=2Pif with respect to the pitch Pmg of the grating mark MG.When the interference fringes are moved in the right direction withrespect to the grating MG in an order shown in FIGS. 13A, 13B, and 13C,the level of the signal I_(m1) sinusoidally changes, as shown in FIG.13D. As shown in FIG. 13B, at a position where each peak of theinterference fringes overlaps the each step edge of the grating mark MG,the signal I_(m1) has a bottom level, as indicated by a point B. Thelevel at a point A in FIG. 13D represents the positional relationship inFIG. 13A, and the level at a point C represents the positionalrelationship in FIG. 13C.

As described above, the level of the signal I_(m1) periodically changesevery time the interference fringes and the grating mark MG are moved byPmg/2 in the X direction. For this reason, unless the interferencefringes and the grating mark MG are slightly moved preliminarily, thepeak and bottom levels of the detected signal I_(m1) cannot be obtained.For this reason, the changes in level of the signal I_(m1) during therelative movement between the interference fringes and the grating markMG by a distance about 5 to 10 times the pitch of the grating MG arestored in the waveform memory circuit 54 in the circuit unit CU₃.

In the circuit unit CU₃, the signal waveform stored in the waveformmemory circuit 54 is arithmetically processed by the calculation circuit56 to obtain an X-direction position offset amount ΔX between theinterference fringes and the grating mark MG on the basis of theamplitude of the signal I_(m1) and a conversion formula F(I_(m1)). Thisposition offset amount ΔX is obtained as a value within the range of±Pmg/4 using the peak or bottom point of the signal I_(m1) in FIG. 13Das a reference (origin).

The function (or formula) F(I_(m1)) is a sine or cosine function becausethe signals I_(m1) is sinusoidal. A radian ψ₁ satisfying the followingcondition:

(E _(p1) +E _(b1))/2+[(E _(p1) −E _(b1))sin ψ₁]/2=e1

where E_(p1) is the peak level of the signal I_(m1), E_(b1) is itsbottom level, and e1 is the level of the signal I_(m1) at a position tobe detected, and a substitution of the radian ψ₁ into the followingconversion formula yields the offset amount ΔX from the reference point:

ΔX=Pmg·ψ/4π

The calculated offset amount ΔX is the position offset amount of thegrating MG to be finally obtained with respect to the grating RG.

In the above embodiment, in irradiating the grating marks RG and MG withthe multi-wavelength illumination beam having the beams LB₁, LB₂, andLB₃ having three different wavelengths at a predetermined lightintensity ratio, the center wavelengths λ₁, λ₂, and λ₃ of the threebeams satisfy relation 1/λ₁−1/λ₂=1/λ₂−1/λ₃ (±10%) The position detectionresults having a higher reliability can be obtained with an effect ofthe multi-wavelength interference beam BM to be photoelectricallydetected. In the optical arrangement in FIG. 12, assume that the gratingRG serves as a grating mark on the mask, that the grating MG serves as amark on the wafer, and that the imaging systems G1 and G2 are projectionlenses for projecting the mask pattern on the wafer. In this case, analignment device in the projection exposure apparatus can be realized.

FIG. 14 shows the&gchematic arrangement according to the thirdembodiment. The same reference numerals as in FIG. 12 denote the samemembers and beams in FIG. 14. In the third embodiment, threeillumination beams LB₁, LB₂, and LB₃ are incident on a small mirror MR2located at the center of a Fourier transform plane EP of an imagingoptical system (G1 and G2) through a lens system G4. The beams LB₁, LB₂,and LB₃ deflected by this small mirror MR2 are converted into parallelbeams through a rear-group lens system G2, and are vertically irradiatedon a grating mark MG.

First-order diffracted beams ±D₁₁ of a wavelength λ₁, 1st-orderdiffracted beams ±D₁₂ of a wavelength λ₂, and 1st-order diffracted beams±D₁₃ of a wavelength λ₃, all of which are diffracted by the grating markMG, and crossed (imaged) on a grating RG through the lens systems G1 andG2. Since the grating RG is of a transmission type, the ±1st-orderdiffracted light components of all the rediffracted light componentsgenerated by the grating RG upon irradiation of the 1st-order diffractedbeams ±D₁₁ serve as an interference beam B_(m1) (including BM), and thisbeam propagates in a direction opposite to the imaging optical systemsand perpendicular to the grating RG. The interference beam B_(m1)reaches a spatial filter 28 through a mirror MR3 and a lens system G5.The spatial filter 28 removes unnecessary diffracted components, and theresultant beam is received by a photoelectric element DT.

The ±1st-order diffracted light components of the rediffracted lightcomponents generated by the grating RG upon irradiation of the 1st-orderdiffracted beams ±D₁₂ and ±D₁₃ having the wavelengths λ₂ and λ₃ areconverted into interference beams B_(m2) and B_(m3) (including BM) whichthen vertically propagate from the grating RG. These beams are receivedby the photoelectric element DT through the mirror MR3, the lens systemG5, and the spatial filter 28. These interference beams B_(m1), B_(m2),and B_(m3) are coaxially generated as one interference beam BM. However,since separate light sources are arranged for generating the respectivebeams, the interference beams B_(m1, B) _(m2), and B_(m3) will notinterfere with each other.

This embodiment has a relationship between beam incidence and receptionwhich is reverse to that in FIG. 12. This arrangement can be applied toan apparatus in which the grating mark MG is formed on a semiconductorwafer, the grating mark RG is formed on a reticle (mask), and the lenssystems G1 and G2 are reduction projection lenses for projecting andexposing the reticle pattern, as disclosed in Japanese PatentApplication Laid-open No. 3-3223 (corresponding U.S. Pat. No. 5,100,237;to be referred to as reference (F) hereinafter).

In the apparatus disclosed in reference (F), a small lens for deflecting1st-ordered diffracted beams by a small amount is arranged on a pupilplane EP of a projection lens to correct the chromatic aberrationgenerated by the projection lens. When the embodiment of FIG. 14 isapplied to this apparatus, a small lens (e.g., flint glass having alarge color dispersion) for optimally correcting the respective1st-order diffracted beams ±D₁₁, ±D₁₂, and ±D₁₃ (three pairs ofillumination beams for forming two-beam light) having small wavelengthdifferences. In the third embodiment, the illumination beams LB₁, LB₂,and LB₃ are directly incident on, e.g., the grating mark MG on thewafer. For this reason, the intensities of the 1st-ordered diffractedbeams ±D₁₁, ±D₁₂, and D₁₃ generated from the grating mark MG can be sethigher than the intensities of the diffracted beam (interference beamBM) generated by the grating mark MG in FIG. 12 as a whole.

When a fiducial mark plate FG having a chromium surface having a knownreflectance is fixed on a wafer stage WST, this mark plate FG can beused for measuring various baseline amounts and focus states. Thebaseline amount basically means an actually measured value fordetermining the relative positional relationship between the projectionpoint of the center of a mask (reticle) mounted in the projectionexposure apparatus and the detection center point in each of the variousalignment systems.

FIG. 15 shows the schematic arrangement of an alignment system in aprojection exposure apparatus which requires baseline amount measurementas the fourth embodiment of the present invention. A reticle R ischucked on a reticle stage RST by vacuum suction and is uniformlyirradiated with an ultraviolet ray (e.g., i-line or excimer laser beam)emitted from an exposure illumination system ILX through a dichroicmirror DCM. The pattern image of the reticle R is projected and exposedin a predetermined shot region on the wafer W through a one-to-one orreduction projection optical system PL.

Referring to FIG. 15, marks detectable by an alignment system TTRA of athrough-the-reticle (TTR) scheme, marks detectable by an alignmentsystem TTLA of a through-the-lens (TTL) scheme, and marks detectable byan alignment system OFA of an off-axial scheme fixed outside theprojection optical system PL are formed on the surface of the fiducialmark plate FG on the wafer stage WST.

Some of these marks are commonly used. An alignment system RA and thealignment systems TTRA, TTLA, and OFA have detection center pointsR_(f1), R_(f2), R_(f3), and R_(f4) serving as direct or indirectreferences in mark detection.

When the position detection apparatus shown in FIG. 6 is applied to eachalignment system, the detection center points R_(f1), R_(f2), R_(f3),and R_(f4) are defined by a reference grating SG. In the reticlealignment system RA, when a reticle alignment mark (grating pattern) RMin the peripheral portion of the reticle R and a corresponding gratingmark on the fiducial mark plate FG are-irradiated with illuminationlight having the same wavelength as that of the illumination light forprojecting and exposing a pattern PR, and these two marks finely movethe reticle stage RST so that the two marks have a predeterminedpositional relationship, the detection center point R_(f1) need not beused.

This also applies to the alignment system TTRA. When the correspondingmark on the fiducial mark plate FG or the mark on the wafer, and adie-by-die (D/D) alignment mark formed in the peripheral portion of thepattern PR of the reticle R are imaged, and a position offset betweenthese two mark images is detected, the detection center point R_(f2)need not be defined.

The baseline amounts indicate the X-Y positional relationship betweenthe projection point (substantially coinciding to an optical axis AX) ofa center CCr of the reticle R on the wafer and the projection points ofthe detection center points R_(f1), R_(f2), R_(f3), and R_(f4) on thewafer. This positional relationship can be obtained by causing thealignment systems RA, TTRA, TTLA, and OFA to detect the position offsetamounts between the corresponding marks on the fiducial mark plate FGand the projection points of the detection center points R_(f1) toR_(f4), and at the same time causing a laser interferometer 44 (see FIG.6) to detect the corresponding coordinate position of the wafer stageWST.

When multi-wavelength incident beams ±LF are used, the pitches and therelative phases,; in the pitch direction, of the interference fringesgenerated on the wafer upon radiation of incident beams of therespective wavelength components are slightly different from each other(e.g., about 0.05 μm). Such a small offset can be eliminated to beideally zero by the fine adjustment operation. Such time-consumingadjustment is not practical because a drift occurs over time. Instead, acalibration function of actually measuring the offsets in relativeposition (phase) for each wavelength component of the multi-wavelengthinterference fringes, i.e., the color offset of the interference fringesis preferably incorporated. This function will be described in detaillater.

The pupil plane EP in the projection optical system PL shown in FIG. 15is identical to the Fourier transform plane EP shown in FIG. 12. Theoptical axes of objective lenses arranged in the alignment systems RA,TTRA, and TTLA for detecting objects (the mark on the wafer W and themark of the fiducial mark plate FG) on the wafer stage WST through theprojection optical system PL are almost parallel to the optical axis AXon the wafer stage WST side.

When the reticle side of the projection optical system PL as well as itswafer side is set telecentric (FIG. 15), the optical axes of theobjective lenses of the alignment systems are set parallel to theoptical axis AX of the projection optical system PL. The extended linesof the optical axes of the objective lenses pass the center (a portionthrough which the optical axis AX passes) of the pupil plane EP of theprojection optical system PL. The effective radius of the pupil plane EPcorresponds to the numerical aperture (NA) which determines theresolving power (minimum resolution line width) of the projection lensPL. A projection lens having NA=about 0.5 to 0.7 is being developed atpresent.

FIG. 16 shows the main part of the alignment system TTLA of all thealignment systems shown in FIG. 15. The pair of multi-color incidentbeams ±LF (corresponding to the beam +LF and the beam −LF in FIG. 6) fordetecting the grating mark MG on the wafer or the fiducial mark plate FGare incident on the projection lens PL through a correction opticalsystem CG, a polarizing beam splitter PBS (functionally corresponding tothe half mirror 20 in FIG. 4), a λ/4 plate QW, an-objective-lens OBJ(corresponding to the objective lens 22 in FIG. 6) and two mirrors MR.

In this case, a plane FC conjugate to the surface of the wafer W isformed between the two mirrors MR. The pair of beams ±LF cross on thisplane FC. The beams ±LF are relayed by the projection lens PL and alsocross on the wafer, thereby irradiating the grating mark MG.

In this embodiment, the beams ±LF incident on the polarizing beamsplitter PBS are linearly polarized beams, and the incident beamefficiently reflected by the polarizing beam splitter PBS is convertedinto a circularly polarized beam rotated in one direction upon passingthrough the λ/4 plate QW. The beam then irradiates the grating mark MGon the wafer through the objective lens OBJ and the projection lens PL.

The interference beam BM vertically generated from the grating mark MGpasses substantially through the center of the pupil plane EP of theprojection lens PL and reaches the polarizing beam splitter PBS throughthe two mirrors MR, the objective lens OBJ, and the λ/4 plate QW. Atthis time, the interference beam BM is a linearly polarized beam in adirection perpendicular to the polarization direction of the incidentbeam, efficiently passes through the polarizing beam splitter PBS, andreaches a photoelectric element 36A.

In this alignment system TTLA, when the incident beams ±LF include aplurality of wavelength components (these components are separated fromeach other by about 30 nm to 40 nm), the crossing region of the beams±LF irradiated on the wafer is shifted in the Z direction or the X and Ydirections due to the influence of the chromatic aberration (on-axialfactor and magnification factor) or the influence-of the chromaticaberration of the objective lens OBJ. The correction optical system CGfor correcting the errors generated in accordance with the chromaticaberrations is arranged in the optical path of the incident beams ±LF.This correction optical system CG comprises a convex lens, a concavelens, a combination thereof, oar a plane-parallel glass member.Alternatively, the correction optical system CG may be constituted byadjustment optical systems 14, 16, and 18 shown in FIG. 6.

In the alignment system TTRA in FIG. 15, a D/D alignment mark DDM on thereticle R serves as a diffraction grating. When a relative positionoffset between the mark DDM and the corresponding grating mark MG on thewafer W is to be detected in accordance with the heterodyne scheme shownin FIG. 4, the influences of the chromatic aberration of on-axial andthe chromatic aberration of magnification are reduced as disclosed inJapanese Patent Application Laid-open No. 6-302504 (corresponding U.S.Ser. No.198,077 filed on Feb. 17, 1994; to be referred,to as reference(G) hereinafter). A transparent plane-parallel correction plate PGP isarranged on the pupil plane EP of the projection lens PL, and atransparent phase grating (corrugated lines are obtained by etching onthe surface of the correction plate PGP at a predetermined pitch) isformed on the correction plate PGP at only the position where theincident beams (±LF) and the interference beam (BM) pass.

FIGS. 17A-17D show the arrangement of a projection exposure apparatus inwhich such a correction plate PGP is incorporated in part of analignment system TTRA according to the fifth embodiment. FIG. 17A showsthe optical path of incident beams ±LF and an interference beam BM indetecting a grating mark MG having a pitch in the X direction(measurement direction) along the X-Z plane, while FIG. 17B shows thisoptical path along the Y-Z plane perpendicular to the X-Z plane.

The pair of incident beams ±LF are decentered from an optical axis AXaand emerge from an objective lens OBJ (corresponding to the objectivelens 22 in FIG. 6) of an alignment system TTRA. The beams are reflectedby a mirror MR and incident on a projection lens PL through a window RWaround the pattern region of a reticle R. The pair of incident beams ±LFare multi-wavelength beams. When viewed along the X-Z plane, the beamspass though the window RW with symmetrical inclinations, as shown inFIG. 17A. The beams are inclined with respect to the optical axis AXa ofthe objective lens OBJ when viewed along the Y-Z plane, as shown in FIG.17B.

The pair of incident beams ±LF pass through phase diffraction gratings(to be referred to as phase gratings hereinafter) PG1 and PG2 on thecorrection plate PGP located on the pupil plane EP of the projectionlens PL. At this time, the incident beams ±LF are inclined by the phasegratings PG1 and PG2 by a predetermined amount in predetermineddirections as indicated by broken lines and emerge from the projectionlens PL. The incident beams ±LF are irradiated on the grating mark MG ona wafer W at the symmetrical incident angles when viewed along the X-Zplane, as shown in FIG. 17A. When viewed along the Y-Z plane, the beamsare slightly inclined in the Y direction and incident on the gratingmark MG.

The interference beam BM generated from the grating mark while beingslightly inclined in the Y direction is incident on the projection lensPL again and passes through the pupil plane EP at a position differentfrom those of the phase gratings PG1 and PG2. A phase grating PG3 forinclining the interference beam BM by a predetermined amount in apredetermined direction, as indicated from the broken line to the solidline in FIG. 17B, is arranged at the above position. Therefore, theoptical path of the interference beam BM is corrected to pass throughthe projection lens PL and to be directed toward the window RW of thereticle R.

The interference beam BM passing through the window RW is directedtoward the light reception system as in FIG. 4 through the mirror MR andthe objective lens OBJ. At this time, the interference beam BM passesthrough the window RW of the reticle R while being slightly inclined inthe non-measurement direction with respect to the optical axis AXa ofthe objective lens OBJ.

When this correction plate PGP and the multi-wavelength incident beams±LF are used, the respective wavelength components of the incident beams±LF are located lightly offset from each other on the correction platePGP in the X direction. For this reason, the phase gratings PG1 and PG2are also formed in a large size corresponding to the offsets of thewavelength components in the X direction. Such use of the correctionplate PGP is also possible for an alignment system TTLA shown in FIG.16. For example, in an exposure apparatus using a projection lens (acombination of a reflecting element and a refractive lens can be used)in which quartz or fluorite is used as a refractive lens material and anultraviolet ray (e.g., an excimer laser beam) having a wavelength of 180nm to 300 nm is used as exposure light, the chromatic aberrations forthe wavelengths of beams from the He—Ne laser and a semiconductor laserare very large. A wafer conjugate plane FC shown in FIG. 16 is separatedfrom the projection lens by several tens cm. For this reason, thecorrection plate PGP is used such that the wafer conjugate plane FC onwhich the incident beams ±LF cross comes close to the projection lens.

As described above, the multi-wavelength beams +LF and −LF pass throughthe incident phase gratings PG1 and PG2 on the correction plate PGP. Atthis time, it is difficult to optimize the grating structure of thephase gratings PG1 and PG2 for all the wavelength components to be used.For this reason, the grating structure of the phase gratings PG1 and PG2is optimized for specific wavelength components. Adjustment opticalmembers are preferably arranged in the incident optical path (generallyon the light source side from the objective lens OBJ) of the incidentbeams ±LF to compensate for only the directional and positionaldifferences, in advance, caused by differences in diffraction behaviorsacting on the respective wavelength components of the incident beams inthe phase gratings PG1 and PG2.

More specifically, it is important to adjust the positions of adjustmentoptical systems 14, 16, and 18 or the position of a correction lens CGin FIG. 16 so as not to cause extreme position and pitch offsets., inunits of wavelength components, of the interference fringes formed onthe grating mark MG of the wafer W (or the fiducial mark plate FG) uponinterference of the pair of incident beams ±LF.

The sixth embodiment of the present invention will be described withreference to FIG. 18. In this embodiment, the polarization directions ofa pair of incident beams +LF and −LF for irradiating a measurement(alignment) grating mark MG on a wafer W through an objective lens 22are set complementary. More specifically, if the incident beams +LF and−LF are linearly polarized beams, their polarization directions are setto be perpendicular to each other. However, if the incident beams +LFand −LF are circularly polarized beams, they are set to be polarizedbeams having reverse rotational directions. For this reason, the twoincident beams ±LF do not interfere with each other, and ±1st-orderpolarized light components BM of wavelengths λ₁, λ₂, and λ₃ verticallygenerated from the grating mark MG do not interfere with each other.When the ±1st-order diffracted light components BM are to bephotoelectrically detected through the objective lens 22 and a smallmirror MR2, a polarizing beam splitter PBS serving as an analyzer isused. In this manner, the ±1st-order polarized: components BM passingthrough the polarizing beam splitter PBS interfere with each other andserve as a first interference beam B_(p1). The ±1st-order diffractedlight components BM reflected by the polarizing beam splitter PBSinterfere with each other and serve as a second interference beamB_(p2).

These interference beams B_(p1) and B_(p2) are complementary. In theheterodyne scheme, the interference beams are sinusoidallyintensity-modulated in accordance with the beat frequency. The intensitymodulation phases of the interference beams B_(p1) and B_(p2) aredifferent by accurately 180°.

When the linear polarization directions of the incident beams ±LF andthe ±1st-order diffracted light component BM which are perpendicular toeach other are different (rotated) from the polarization separationdirection of the polarizing beam splitter PBS, a λ/2 plate HW shown inFIG. 18 is arranged to correct the linear polarization directions of the±1st-order diffracted light beams EM. For this reason, when the linearpolarization directions of the +1st-order diffracted light components BMwhich are perpendicular to each other coincide with the polarizationseparation direction of the polarizing beam splitter PBS from thebeginning, or when the incident beams +LF and −LF are circularlypolarized beams having opposite rotational directions, the λ/2 plate HWneed not be used. In this embodiment, the interference beam B_(p1) isreceived by a photoelectric element 36A₁ through a mirror 32, and theinterference beam B_(p2) is received by a photoelectric element 36A₂through another mirror 32. In addition, output signals I_(a1) and I_(a2)from the photoelectric elements 36A₁ and 36A₂ are subtracted by adifferential amplifier, thereby obtaining a photoelectric signal I_(m1).

The differential amplifier is used as described above because the outputsignal from the photoelectric element 36A₁ has a phase opposite (adifference of 180°) to that of the output signal from the photoelectricelement 36A₂. An in-phase noise component (common-mode noise) includedin both the outputs is canceled by the above subtraction. A substantialS/N ratio of the signal I_(m1) can be increased.

It is preferable that at least an on-axial chromatic aberration of thevarious chromatic aberrations be corrected for the objective lens 22shown in FIG. 18 of this embodiment or in FIG. 6, or an objective lensOBJ shown in FIG. 16 to some extent. If the bandwidth of wavelengths λ₁,λ₂, and λ₃ to be used is 100 nm or less, such an on-axial chromaticaberration can be corrected to some extent by selecting proper materialsfor a plurality of lens elements constituting the objective lens 22 orcombining lens elements having different refractive indices anddifferent dispersion ratios. This chromatic aberration need not beperfectly corrected in the objective lenses 22 and OBJ. The chromaticaberration can be corrected by adjustment optical systems 14, 16, and 18shown in FIG. 6 or a correction optical system CG shown in FIG. 16.

The seventh embodiment of the present invention will be described withreference to FIG. 19. This embodiment discloses a position detectionapparatus added with the following new function. A multi-wavelengthinterference beam BM (±1st-order diffracted light components) generatedfrom a grating mark MG on a wafer W or a fiducial plate FG isphotoelectrically detected in units of wavelength components, and offsetamounts caused by the colors of the interference fringes in units ofwavelengths generated by two-beam interference are automaticallymeasured.

The arrangement of this embodiment shown in FIG. 19 is part of thearrangement in FIG. 6. More specifically, the arrangement in FIG. 19 isobtained by changing a photoelectric detection system for detecting aninterference beam BM from the grating mark MG. The same referencenumerals as in FIG. 6 denote the same parts in FIG. 19. An incidentsystem 100 in FIG. 19 includes light sources LS₁, LS₂, and LS₃, a mirrorMR, dichroic mirrors DCM₄ and DCM₅, a radial grating plate RRG servingas a frequency shifter, a lens 10, a spatial filter 12, and adjustmentoptical systems 14, 16, and 18. The incident system 100 in FIG. 19 emitsa pair of incident beams +LF and −LF.

The incident beams ±LF having wavelengths λ₁, λ₂, and λ₃ are partiallyreflected by a half mirror 20 and incident on an objective lens 22. Theremaining part of the beams is incident on a reference light receptionsystem 110. The reference light reception system 110 comprises awavelength selection filter 24, a lens 26, a reference grating SG, and aspatial filter 38 in FIG. 6. The reference light reception system 110guides a reference light beam B_(ms) to a photoelectric element 40.

When the grating MG on the wafer W is irradiated with the incident beams±LF through the objective lens 22, the interference beam BM of the±1st-order diffracted light components is vertically generated from thegrating MG. At the same time, interference beams of the 0th- and2nd-order diffracted light components are generated in a directionopposite to the traveling direction of each incident beam. Theseinterference beams pass through the objective lens 22, the half mirror20, and reach a spatial filter 28 located on the spatial filter 28 ofthe objective lens 22. Only the interference beam BM of the ±1st-ordereddiffracted light components is selectively transmitted through thespatial filter 28 and is split into two beams by a beam splitter 29.

The interference beam BM reflected by this beam splitter 29 is receivedby a photoelectric element 36A through a lens system 30 as in FIG. 6.The interference beam BM passing through the beam splitter 29 isincident on a spectral detection system 34. The spectral detectionsystem 34 comprises a dichroic mirror DCM₁ for reflecting only aninterference beam B_(m1) of the wavelength λ₁ from the interference beamBM condensed by a lens system 31 and guiding the reflected beam to aphotoelectric element DT₁, and a dichroic mirror DCM₂ for reflectingonly an interference beam B_(m2) of interference beams B_(m2)(wavelength λ₂) and B_(m3) (wavelength λ₃) passing through the dichroicmirror DCM₁ and guiding the reflected interference beam B_(m2) to aphotoelectric element DT₂, and transmitting only the interference beamB_(m3) and guiding the transmitted beam B_(m3) to a photoelectricelement DT₃.

The photoelectric elements DT₁, DT₂, and DT₃ output photoelectricsignals I_(p1), I_(p2), and I_(p3) whose levels sinusoidally change witha beat frequency 2Δf. These photoelectric signals I_(pn) (n=1, 2, 3) areinput to signal processing circuits 70A, 70B, 70C, and 70D shown in FIG.20 together with a photoelectric signal I_(ms) from the photoelectricelement 40 which receives the reference light beam B_(ms). Each signalprocessing circuit includes an A/D-converter 50, a waveform memorycircuit 54, and a calculation circuit 56 shown in FIG. 6 and calculatesa phase offset Δψ for each photoelectric signal. The phase offset Δψinformation of each calculated photoelectric signal is processed by acomputer in a phase offset calculation circuit 72 to obtain a smallposition offset amount between the interference fringes for therespective wavelength components, i.e., the mutual offset amountsassociated with the respective interference fringes in the pitchdirection.

With this arrangement, the A/D converters 50 and the waveform memorycircuits 54 in the signal processing circuits 70A to 70D digitallysample the waveforms of the respective photoelectric signals at the sametiming in response to a clock pulse C_(ps) from a common sampling clockgenerator 52 (FIG. 6). The calculation circuits 56 in the signalprocessing circuits 70A to 70D perform Fourier integration on the basisof prestored sine wave data sinx ωt and prestored cosine wave data cosωt to calculate phase differences Δψ_(p1), Δψ_(p2), Δψ_(p3), and Δψ_(ms)with reference to the data sin ωt (or cos ωt) of the respectivephotoelectric signals I_(p1), I_(p2), I_(p3), and I_(ms). Note that theangular velocity ω of the sine or cos wave data is defined as ω=2π(2Δf)in association of the beat frequency 2Δf.

The position offset calculation circuit 72 calculates position offsetamounts ΔX₁, ΔX₂, and ΔX₃ of the grating mark MG in units of wavelengthson the basis of the phase differences Δψ_(p1), Δψ_(p2), Δψ_(p3), andΔψ_(ms)).

ΔX ₁ =k(Δψ_(ms)−Δψ_(p1))

ΔX ₂ =k(Δψ_(ms)−Δψ_(p2))

ΔX ₃ =k(Δψ_(ms)−Δψ_(p3))

for k=±Pmg/4π

The resultant position offset amounts must be almost equal to valueswhen the grating mark MG on the fiducial plate FG shown in FIG. 6 or 15is detected. If a difference is present between the position offsetamounts ΔX₁, ΔX₂, and ΔX₃, it indicates that offsets are present betweenthe interference fringes for the respective wavelengths in the pitchdirection. For this reason, the grating mark MG on the fiducial plate FGis often detected prior to the alignment of the wafer W to confirm themutual offsets between the interference fringes for the respectivewavelengths. If the mutual offsets are larger than the upper limit ofthe allowable range, the adjustment optical systems 14, 16, and 18 inFIG. 6 and part of a correction optical system CG in FIG. 16 are finelymoved to set the mutual offsets to fall within the allowable range.

In this embodiment, the interference fringes generated by the twomulti-wavelength beams can be always maintained to be free from themutual offsets, thereby reducing the error in detecting the grating markon the wafer W.

In the first to seventh embodiments, three or more illumination beamsfrom a plurality of highly coherent light sources are synthesized toobtain a multi-wavelength illumination light beam and detect a periodicpattern such as a grating mark upon irradiation of this illuminationlight beam. In this case, the wavelengths of the three or moreillumination beams for obtaining the multi-wavelength illumination beamare set such that difference values of wave numbers (1/λ_(n)) betweenany two adjacent illumination beams along the wavelength axis fallwithin the allowable range of ±10%. Therefore, even if highly coherentlight sources are used, a highly incoherent arrangement can be achieved.

For this reason, position detection of a grating mark or the like can beperformed with a higher precision than the conventional case. Inaddition, the position detection apparatus illustrated in eachembodiment described above is applicable not only to the projectionexposure apparatus, but also to various alignment systems in proximityexposure apparatuses.

Each embodiment associated with photoelectric processing in amulti-wavelength alignment system in association with the second aspectof the present application will be described below.

FIG. 21 shows the arrangement of a position detection apparatusaccording to the eighth embodiment of the present invention. Thisarrangement is basically similar to that in FIG. 12. The eighthembodiment exemplifies homodyne measurement of a relative positionoffset between two diffraction gratings RG and MG in the pitch direction(X direction). Beams LB₁ and LB₂ serving as illumination light beams areemitted from different laser light sources at different wavelengths λ₁and λ₂. These beams are coaxially synthesized, and the resultant beam isvertically irradiated on the grating RG through a beam splitter BS and amirror MR1. The beam splitter BS divides the part (about several %) ofthe amplitudes of the beams LB₁ and LB₂ and guides them to photoelectricelements DT₁ and DT₂ through a dichroic mirror DCM₁. The dichroic mirrorDCM₁ transmits 90% or more of the beam LB₁ having the wavelength λ₁ andguides it to the photoelectric element DT₁, and at the same timereflects 90% or more of the beam LB₂ having the wavelength λ₂ and guidesit to the photoelectric element DT₂. The photoelectric elements DT₁ andDT₂ output a signal I_(r1) representing the intensity value of thereceived beam having the wavelength λ₁ and a signal I_(r2) representingthe intensity value of the received beam having the wavelength λ₂.

A plurality of diffracted light components are generated from thegrating RG upon irradiation of the beams LB₁ and LB₂ (parallel beams).If the grating RG is a one-dimensional transmission grating having aduty of 1:1, and its pitch direction is the horizontal direction on thedrawing surface in FIG. 21, these diffracted light components(diffracted beams) are deflected at predetermined diffraction angleswithin the drawing surface in FIG. 21.

FIG. 21 shows 1st-order diffracted beams +D₁₁ and −D₁₁ generated by thebeam LB₁ having the wavelength λ₁, 1st-order diffracted beams +D₁₂ and−D₁₂ generated by the beam LB₂ having the wavelength λ₂, and a 0th-orderdiffracted beam D₀. Other diffracted components of higher order are alsogenerated for the beams LB₁ and LB₂. For the sake of descriptivesimplicity, only the 1st-order diffracted beams are illustrated.

Each diffracted beam is incident on an imaging optical system dividedinto the front-group lens system G1 and the rear-group lens system G2.When the grating RG is located at the position of a front focal lengthf1 a of the front-group lens system G1, and the position of a rear focallength f1 b of the front-group lens system G1 coincides with theposition of a front focal length f2 b of the rear-group lens system G2to form a Fourier transform plane EP, the respective 1st-orderdiffracted beams cross (imaging) at the position of a rear focal lengthf2 a of the rear-group lens system G2. Note that the chromaticaberrations of the lens systems G1 and G2 have been corrected.

As shown in FIG. 21, a small mirror MR2 is fixed at the center of theFourier transform plane (pupil plane) EP. The 0th-order beam D₀ from thegrating RG is shielded by this mirror MR2 and prevented from beingincident on the rear-group lens system G2. Each 1st-order diffractedbeam emerges from the grating RG, it is a parallel beam as in the beamsLB₁ and LB₂. However, each 1st-order diffracted beam is converged as abeam waist at the position of the Fourier transform plane EP by thebehavior of the front-group lens system G1.

If the pitch of the grating RG is defined as Prg, a diffraction angle θ₁(an angle with respect to the 0th-order diffracted beam) of the1st-order diffracted beams ±D₁₁ generated by the beam LB₁ having thewavelength λ₁ and a diffraction angle θ₂ of the 1st-order diffractedbeams ±D₁₂ generated by the beam LB₂ having the wavelength λ₂ are givenby the following equations:

sin θ₁=λ₁ /Prg  (5)

sin θ₂=λ₂ /Prg  (6)

In this case, if λ₁<λ₂, then θ₁<θ₂ is established. On the Fouriertransform plane EP, the 1st-order diffracted beams ±D₁₁ pass inside (onthe 0th-order beam D₀ side) the 1st-order diffracted beams ±D₁₂.

The respective 1st-order diffracted beams are converted into parallelbeams and superposed on each other on the reflection grating MG formedby corrugations on the object side through the rear-group lens G2. Atthis time, the pitch direction of the grating MG coincides with the Xdirection. One-dimensional interference fringes of the wavelength λ₁(the pitch direction is the X direction) are generated by two-beaminterference of the 1st-order diffracted beams ±D₁₁, and one-dimensionalinterference fringes of the wavelength λ₂ (the pitch direction is the Xdirection) are generated by two-beam interference of the 1st-orderdiffracted beams ±D₁₂. At this time, since the light beam having thewavelength λ₁ and the light beam having the wavelength λ₂ have differentwavelengths, no interference occurs between the 1st-order diffractedbeams ±D₁₁ and ±D₁₂. It is important that the interference fringeshaving the wavelengths λ₁ and λ₂ generated by the 1st-order diffractedbeams ±D₁₁ and ±D₁₂ appear as one set of interference fringes having thesame intensity distribution pitch.

An intensity distribution pitch Pif of the interference fringes isdetermined by the pitch Prg of the grating mark RG and a magnificationfactor M of the imaging optical system (G1 and G2) and is defined asPif=M·Prg/2. For example, when the pitch Prg is set to be 4 μm, and themagnification factor M is ¼ (the pattern size of the grating RG isreduced into ¼ on the grating mark MG side), the pitch Pif of theinterference fringes becomes 0.5 μm. If the pitch Pmg of the grating MGto be measured is given as Pmg=2Pif, i.e., Pmg=M·Prg, rediffracted lightcomponents are generated from the grating MG using the 1st-orderdiffracted beams ±D₁₁ as the incident beams. For example, onerediffracted light component generated by the grating mark MG uponirradiation of the 1st-order diffracted beam +D₁₁ as the incident beamis the −1st-order diffracted light component (wavelength λ₁) verticallypropagating from the grating mark MG. One rediffracted light componentgenerated from the grating mark MG upon irradiation of the 1st-orderdiffracted beam −D₁₁ as the incident beam is the +1st-order diffractedlight component (wavelength λ₁) vertically propagating from the gratingmark MG. The ±1st-order diffracted light components of the wavelength λ₁vertically propagating from the grating mark MG have an interferenceintensity corresponding to the mutual phase states, serve as aninterference beam BM, and reach the mirror MR2.

Similarly, rediffracted light components are generated from the gratingmark MG upon irradiation of the 1st-order diffracted beams ±D₁₂ as theincident beams. The −1st-order diffracted light component of (wavelengthλ₂) generated from the grating mark MG upon irradiation of the 1st-orderdiffracted beam +D₁₂ propagates in a direction perpendicular to thegrating mark. The +1st-order diffracted light component (wavelength λ₂)generated from the grating mark MG upon irradiation of the 1st-orderdiffracted beam −D₁₂ propagates in a direction perpendicular to thegrating MG. The ±1st-order diffracted light components of the wavelengthλ₂ propagating in the direction perpendicular to the grating mark MGalso have an interference intensity corresponding to the mutual phasestates, serve as an interference beam MB, and reach the mirror MR2. Theinterference beam BM coaxially includes an interference beam B_(m1)having the wavelength λ₁ and an interference beam B_(m2) having thewavelength λ₂.

The interference beam BM is reflected by the mirror MR2 and reachesphotoelectric elements DT₃ and DT₃ through a lens system G3 constitutinga photoelectric detection system, and a dichroic mirror DCM₂. Thisdichroic mirror DCM₂ divides the wavelengths λ₁ and λ₂ and issubstantially identical to the dichroic mirror DCM₁. The interferencebeam B_(m1) having the wavelength λ₁ in the interference beam BM isreceived by the photoelectric element DT₃, while the interference beamB_(m2) having the wavelength λ₂ in the interference beam MB is receivedby the photoelectric element DT₄.

The photoelectric element DT₃ outputs a photoelectric signal I_(m1)having a level corresponding to the intensity of the interference beamB_(m1) to circuit units CU₁ and CU₃. The photoelectric element DT₄outputs a photoelectric signal I_(m2) having a level corresponding tothe intensity of the interference beam B_(m2) to circuit units CU₂ andCU₄. The circuit unit CU₁ calculates a ratio C₁ of the amplitude valueof the photoelectric signal I_(m1) to that of the signal I_(r1) from thephotoelectric element DT₁ in accordance with a calculation ofI_(m1)/I_(r1). The circuit unit CU₂ calculates a ratio C₂ of theamplitude value of the photoelectric signal I_(m2) to that of the signalI_(r2) from the photoelectric element DT₂ in accordance with acalculation of I_(m2)/I_(r2). The ratios C₁ and C₂ are output to acircuit unit CU₅ for calculating a weighted mean (to be describedlater).

Since the homodyne scheme is employed in this embodiment, theintensities of the interference beams B_(m1) and B_(m2) change inaccordance with relative changes in positions of the gratings RG and MGin the X direction. If the gratings RG and MG are kept stopped, thelevels of the signals I_(m1) and I_(m2) continuously have predeterminedvalues. The grating mark MG and the interference fringes generated bythe grating mark RG on the grating mark MG are scanned relative to eachother by a predetermined amount (the pitch Pif or more of theinterference fringes) in the X direction, and the peak values and thebottom values during the sinusoidal changes in levels of the signalsI_(m1) and I_(m2) generated during scanning are sampled. The differencevalues between the peak and bottom values are used for arithmeticoperations in the circuit units CU₁ and CU₂.

The level change in the signal I_(1m) (this also applies to the signalI_(m2)) corresponding to the change in positional relationship betweenthe interference fringes and the grating MG will be described withreference to FIGS. 22A to 22D. FIGS. 22A, 22B, and 22C are identical toFIGS. 13A, 13B, and 13C. The interference fringes having the pitch Pifhave a good sinusoidal intensity distribution due to two-beaminterference and are set to be Pmg=2Pif with respect to the pitch Pmg ofthe grating mark MG. When the interference fringes are moved in theright direction with respect to the grating MG in an order shown inFIGS. 22A, 22B, and 22C, the level of the signal I_(m1) sinusoidallychanges, as shown in FIG. 22D. As shown in FIG. 22B, at a position whereeach peak of the interference fringes overlaps the each step edge of thegrating mark MG, the signal I_(m1) has a bottom level, as indicated by apoint B. The level at a point A in FIG. 22D represents the positionalrelationship in FIG. 22A, and the level at a point C represents thepositional relationship in FIG. 22C.

As described above, the level of the signal I_(m1) periodically changesevery time the interference fringes and the grating mark MG are moved byPmg/2 in the X direction. For this reason, unless the interferencefringes and the grating mark MG are slightly moved preliminarily, thepeak and bottom levels of the detected signal I_(m1) cannot be obtained.This is also true for the signal I_(m2). Since the signal I_(m2)represents the intensity of the interference beam B_(m2) of the±1st-order diffracted light components, the phase of the signal I_(m2)is not extremely offset from that of the signal I_(m1) (an offset ofabout several % may occur depending on the resist interference and markasymmetry), although the level of the signal I_(m2) greatly differs fromthat of the signal I_(m1), as indicated by an imaginary line in FIG.22D. For this reason, at an arbitrary positional relationship that theinterference fringes and the grating MG are kept stopped, even if thelevels of the signals I_(m1) and I_(m2) are sampled, the ratios C₁ andC₂ can be theoretically calculated by the circuit units CU₁ and CU₂.However, as can be apparent from FIG. 22D, the signals I_(m1) and I_(m2)can be advantageously sampled at their peak points in favor of reductionin various noise components and improvement of detection precision.

As shown in FIG. 21, the circuit units CU₃ and CU₄ calculate X-directionposition offset amounts ΔX₁ and ΔX₂ between the interference fringes andthe grating MG on the basis of the amplitude values of the signalsI_(m1) and I_(m2) and preset functions or conversion formulas F(I_(m1))and F(I_(m2)). These position offset values ΔX₁ and ΔX₂ are obtained asvalues within the range of ±Pmg/4 using the peak or bottom values of thesignals I_(m1) and I_(m2) as the references (origins).

The functions (or formulas) F(I_(m1)) and F(I_(m2)) are sine or cosinefunctions because the signals I_(m1) and I_(m2) are sinusoidal. A radianψ₁ is calculated to satisfy the following condition:

(E _(p1) +E _(b1))/2+[(E _(p1) −E _(b1))sin ψ₁]/2=e 1

where E_(p1) is the peak level of the signal I_(m1), E_(b1) is itsbottom level, and e1 is the level of the signal I_(m1) at a position tobe detected, and a substitution of the radian ψ₁ into the followingconversion formula yields the offset amount ΔX₁ from the referencepoint:

ΔX=Pmg·ψ/4π  (7)

The calculated offset amounts ΔX₁ and ΔX₂ are supplied to the circuitunit CU₅ for calculating the weighted mean, and the following arithmeticoperation is performed using the ratios C₁ and C₂ as the weightingfactors.

ΔX=(C ₁ ·ΔX ₁ +C ₂ ·ΔX ₂)/(C ₁ +C ₂)  (8)

The resultant offset amount ΔX is the final position offset value of thegrating MG with respect to the grating RG.

As can be apparent from the above formula, a larger weighting factor isused for the measurement result of the position offset value using aninterference beam of a higher intensity in all the interference beamsBM. As described above, in this embodiment, the beams LB₁ and LB₂ havingdifferent wavelength components are used to irradiate the gratings RGand MG, and the interference beam BM to be received is photoelectricallydetected in units of wavelengths. The position offset resultsrespectively obtained using the interference beams B_(m1) and B_(m2) forthe respective wavelengths are subjected to the weighted meancalculation in accordance with the amplitudes of the received lightbeams of the respective wavelengths. Therefore, a higher-precisionposition detection result can be obtained.

The algorithm of the signal processing systems (circuit units CU₁ toCU₅) shown in FIG. 21 is commonly used in the remaining embodiments. Ifchanges and improvements are made to realize the functions of therespective circuit units, they will be described. In the opticalarrangement shown in FIG. 21, if the grating RG is a grating mark on amask, the grating MG is a mark on a wafer, and the imaging systems G1and G2 are projection lenses for projecting the mask pattern on thewafer, an alignment device in a projection exposure apparatus can berealized.

FIG. 23 shows the schematic arrangement of the ninth embodiment. Thisarrangement is basically or identical to that of FIG. 14 but isdifferent from FIG. 14 in the arrangement of a light reception system.The same reference numerals as in FIG. 14 denote the same members andbeams in FIG. 21. In the ninth embodiment, two illumination beams LB₁and LB₂ are incident on a mirror MR2 located at the center of the pupilplane of an imaging optical system (G1 and G2) through a lens system G4.The beams LB₁ and LB₂ deflected downward by the mirror MR2 are convertedinto parallel beams through the rear-group lens system G2 and verticallyirradiated on a grating MG. First-order diffracted beams ±D₁₁ of awavelength λ₁ diffracted by the grating MG and 1st-order diffractedbeams ±D₁₂ of a wavelength λ₂ diffracted by the grating MG cross(imaging) on a grating RG through the lens system G1 and the lens groupG2. Since the grating RG is of a transmission type, the ±1st-orderdiffracted light components of the rediffracted light beams from thegrating RG upon irradiation of the 1st-order diffracted beams ±D₁₁propagate in a direction opposite to the imaging optical system andperpendicular to the grating RG. The 1st-order diffracted lightcomponents become an interference beam B_(m1) through a mirror MR3 and adichroic mirror DCM₃, and the interference beam B_(m1) is incident on aphotoelectric element DT₃. The ±1st-order rediffracted light componentsgenerated upon irradiation of the 1st-order diffracted beams ±D_(l2)become an interference beam B_(m2). The interference beam B_(m2) passesthrough the same optical path as that of the interference beam B_(m1),is selected by a dichroic mirror DCM₃, and reaches a photoelectricelement DT₄. The remaining arrangement is the same as that of FIG. 21.

This embodiment has the relationship between beam incidence and beamreception which is opposite to that in FIG. 21. In the arrangement ofthis embodiment, the grating MG is formed on a semiconductor wafer, thegrating RG is formed on a reticle (mask), and this arrangement can beapplied the apparatus of reference (F) (Japanese Patent ApplicationLaid-open No. 3-3224) in which lens systems G1 and G2 are reductionprojection lenses for projection exposure. In the apparatus disclosed inreference (F), the small lens for slightly deflecting 1st-orderdiffracted beams on the pupil plane EP of the projection lens, therebycorrecting the chromatic aberration generated by the projection lens.However, when the embodiment of FIG. 23 is applied, a small lens (e.g.,flint glass having a large color dispersion) must be arranged tooptimally correct 1st-order diffracted beams ±D₁₁ and ±D₁₂ having asmall wavelength difference.

In the ninth embodiment, since the illumination beams LB₁ and LB₂ aredirectly incident on, e.g., the grating MG on the wafer, the intensitiesof the 1st-order diffracted beams ±D₁₁ and ±D₁₂ generated from thegrating MG can be set higher than the diffracted beams (interferencebeams BM) generated from the grating MG in FIG. 21.

The 10th embodiment of the present invention will be described withreference to FIGS. 24 and 25 and is basically the same as the apparatusof FIG. 6 except that the arrangement of a light reception system isslightly different. A heterodyne scheme is used in place of a homodynescheme. Referring to FIG. 24, three laser light sources LS₁, LS₂, andLS₃ generate laser beams LB₁, LB₂, and LB₃ having different wavelengthsand λ₁, λ₂, and λ₃, respectively. For example, the laser light sourceLS₁ is an He—Ne laser light source of λ₁=0.633 μm, the light source LS₂is a semiconductor laser light source of λ₂=0.690 μm, and the lightsource LS₃ is a semiconductor laser light source of λ₃=0.760 μm. Thewavelength relationship is given as λ₁<λ₂<λ₃.

These three beams LB₁, LB₂, and LB₃ are synthesized into one coaxialbeam LB₀ through a mirror MR and dichroic mirrors DCM₄ and DCM₅. Thebeam LB₀ is reflected by the mirror MR and incident on a rotary radialgrating plate RRG. As previously shown in FIG. 7, the grating plate RRGis rotated about a rotation axis C₀ at a high constant speed in onedirection. The frequencies of the diffracted lights of respective ordersdiffracted by the grating plate RRG are changed by the grating plate RRGby an amount corresponding to the angular velocity.

In this embodiment, the heterodyne scheme is realized using the±1st-order diffracted light components from the radial grating plateRRG, only ±1st-order diffracted beams ±LF from the grating plate RRG areillustrated in FIG. 24.

First-order diffracted beams ±D₁₁ generated from the beam LB₁ having thewavelength λ₁, 1st-order diffracted beams ±D₁₂ generated from the beamLB₂ having the wavelength λ₂, and 1st-order diffracted beams ±D₁₃generated from the beam LB₃ having the wavelength λ₃ are generated fromthe grating RG of the rotary grating plate RRG in the same manner asshown in FIG. 21. A diffraction angle of the 1st-order diffracted beamsfor each wavelength is represented as follows:

sin θ_(n)=λ_(n) /Prg

where n is the number of wavelengths, and Prg is the pitch of thegrating RG of the rotary grating plate RRG.

Each 1st-order diffracted beam is subjected to a predetermined frequencyshift Δf regardless of the wavelengths. If a velocity at which thegrating RG of the grating plate RRG crosses the beam LB₀ is defined asV, Δf=V/Prg is obtained. A +1st-order diffracted beam has a frequencyhigher than the frequency of the 0th-order diffracted light component byΔf, while a −1st-order diffracted beam has a frequency lower than thefrequency of the 0th-order diffracted light component by Δf. Therefore,the rotary radial grating plate RRG serves as a frequency shifter.

Incident beams tLF consisting of the 1st-order diffracted beams ±D_(1n)(n=1, 2, 3) having the three wavelength components and the 0th-orderdiffracted light component D₀ are converted by a collimator lens 10 suchthat their principal rays are parallel to each other, as shown in FIG.24. These beams reach a beam selection member 12. The beam selectionmember 12 serves as a spatial filter located on the so-called Fouriertransform plane. The beam selection member 12 shields the 0th-orderdiffracted light component D₀ and passes the incident beams ±LF derivedfrom the 1st-order diffracted light components ±D₁.

The incident beams ±LF then reach a beam splitter (half mirror) 20through adjustment optical systems 14, 16, and 18 constituted byplane-parallel glass members whose inclination amounts are variable. Theadjustment optical system 14 has a function of deflecting the incidentbeams ±LF with respect to the optical axis of the lens 10 while thedistance between the incident beams +LF and −LF in the Fourier space iskept unchanged. The adjustment optical systems 16 and 18 have functionsof individually adjusting the incident beams +LF and −LF with respect tothe optical axis.

The incident beams ±LF are split into two pair of beams by the beamsplitter 20. One pair of beams is incident on an objective lens 22,while only the 1st-order diffracted beams having a specific wavelengthin the splitted incident beams ±LF, i.e., only the 1st-order diffractedbeams ±D₁₂ having the wavelength λ₂ of the other beam are selected andincident on a condenser lens (Fourier transform lens) 26.

The incident beams ±LF incident on the objective lens 22 are collimatedinto parallel beams which are then simultaneously irradiated on thegrating MG on a wafer W at different angles. Interference fringes formedby the interference of the incident beams ±D₁₁ having the wavelength λ₁,interference fringes formed by the interference of the incident beams±D₁₂ having the wavelength λ₂, and interference fringes formed by theinterference of the incident beams ±D₁₃ having the wavelength λ₃ aresuperposed on each other and appear on the grating MG at the same pitchand the same phase. The interference fringes are observed as if they aremoving on the grating MG at a constant speed in one direction because ofthe frequency difference 2Δf between the incident beams +LF and −LF.This moving speed is proportional to the velocity V of the grating RG ofthe rotary radial grating plate RRG. As can be apparent from FIG. 24,the surface (grating MG) of the wafer W and the radial grating plate RRGare located conjugate to each other (imaging relationship) by acomposite system of the collimator lens 10 and the objective lens 22.For this reason, the images obtained by the ±1st-order diffracted lightcomponents on the grating RG of the radial grating plate RRG are formedon the grating MG of the wafer W. The images (interference intensitydistribution) ½ the pitch of the grating RG are formed because the0th-order diffracted light component D₀ is shielded. The pitch Pif ofthe interference fringes on the wafer W is ½ the pitch Pmg of thegrating MG.

When the above relationship is satisfied, the 1st-order diffracted lightcomponents are generated vertically from the grating MG upon irradiationof the incident beams ±LF. More specifically, an interference beam BM isobtained by interfering the 1st-order diffracted light componentgenerated upon irradiation of the incident beam +LF and the 1st-orderdiffracted light component vertically generated upon irradiation of theincident beam −LF. This interference beam BM is a beat light beamintensity-modulated with the frequency 2Δf. In this manner, to generatethe ±1st-order diffracted light beams (interference beam BM) in the samedirection, distances DL_(n) of the incident beams ±LF of the respectivewavelengths on the Fourier transform plane from the optical axis aredefined as follows from the another viewpoint:

DL _(n) =F ₀·sin θ_(n) =±F ₀·λ_(n) /Pmg (n=1, 2, 3)

where F₀ is the focal length of the objective lens 22. The distanceDL_(n) for each wavelength can be adjusted by appropriately setting thepitch of the grating RG of the rotary radial grating plate RRG and thefocal length of the collimator lens 10.

The interference fringes formed on the wafer W are imaged as adiffraction image of the grating RG of the radial grating plate RRG. Inprinciple, if the pitch of the interference fringes obtained by one ofthe wavelength components having the three wavelengths λ₁, λ₂, and λ₃ isan integer multiple of the pitch of the grating mark MG, the pitches ofthe interference fringes for the remaining wavelength components havethe same relationship. The interference fringes obtained in units ofwavelength components perfectly coincide and are free from phase offsetsand position offsets. In practice, however, the interference fringesobtained in units of wavelength components are subjected to positionoffsets, phase offsets, and pitch offsets in accordance with the degreeof chromatic aberration of the optical systems such as the objectivelens 22 and the collimator lens 10.

To correct these offsets, the adjustment optical systems 14, 16, and 18in FIG. 24 are used. These optical system 14, 16, and 18 are constitutedby plane-parallel glass members. When a material having a large colordispersion is used as the material of these optical systems, theposition and phase offsets of the interference fringes formed on thewafer in units of wavelength components can be slightly changed.Alternatively, a plane-parallel glass member having a small colordispersion may be combined with a plane-parallel glass member having alarge color dispersion to constitute the adjustment optical systems 14,16, and 18. In this case, the inclination of the plane-parallel glassmember having the large color dispersion is adjusted to correct therelationship between the interference fringes for each wavelengthcomponent. An error in the overall inclination of the incident beams ±LFon the wafer, which is caused by the above inclination adjustment can becorrected by inclination adjustment by the plane-parallel glass memberhaving the small color dispersion.

The interference beam BM vertically generated by the interferencefringes from the illuminated grating MG reaches a spatial filter 28through the objective lens 22 and the beam splitter 20. This spatialfilter 28 is located at or near the Fourier transform plane associatedwith the objective lens 22. In this embodiment, the spatial filter 28has an aperture for passing only the interference beam MB (±1st-orderdiffracted light components). The interference beam MB passing throughthe spatial filter 28 is converted into a parallel beam by a lens system(inverse Fourier transform lens) 30. The wavelength of the parallel beamis selected by a first dichroic mirror 32 and a second dichroic mirror34.

The dichroic mirror 32 reflects 90% or more of a beam B_(m1) of thewavelength λ₁ in the interference beam BM, and the reflected beam isincident on a photoelectric element 36A. The dichroic mirror 34 reflects90% or more of a beam B_(m2) of the wavelength λ₂ passing through thedichroic mirror 32, and the transmitted beam is received by aphotoelectric element 36B. A beam B_(m3) of the wavelength λ₃ in theinterference beam BM passes through the dichroic mirrors 32 and 34 andis received by a photoelectric element 36C. The photoelectric elements36A, 36B, and 36C have the same functions as in the photoelectricelements DT₃ and DT₄ in FIG. 21 except that the interference beamsB_(m1), B_(m2), and B_(m3) to be received are intensity-modulated withthe beat frequency 2Δf. The wavelength division of the dichroic mirrors32 and 34 may be unsatisfactory depending on the intervals between thewavelengths λ₁, λ₂, and λ₃ to be used. For this reason, interferencefilters (narrow bandpass filters) may be respectively arranged just infront of the photoelectric elements 36A, 36B, and 36C. Note that thedichroic mirrors 32 and 34 are identical to dichroic mirrors DCM₅ andDCM₄ on the incident system, as a matter of course.

Photoelectric signals I_(m1), I_(m2), and I_(m3) from the photoelectricelements 36A, 36B, and 36C have waveforms whose levels sinusoidallychange at the same frequency as the beat frequency 2Δf in the presenceof the interference beam BM from the grating mark MG.

The 1st-order diffracted beams ±D₁₂ selected by the wavelength selectionfilter 24 and the condenser lens 26 are superposed on each other andirradiated on a transmission reference grating SG. In this case, thereference grating SG is located conjugate to the rotary radial gratingplate RRG (frequency modulator) in association with the composite systemof the collimator lens 10 and the condenser lens 26. Therefore,one-dimensional interference fringes caused by 2-beam interference ofthe 1st-order diffracted beams ±D₁₂ are also formed on the referencegrating SG and move at a speed corresponding to the beat frequency 2Δf.

When the pitch of the reference grating SG and the pitch of theinterference fringes are appropriately determined, the ±1st-orderdiffracted light components generated from the reference grating SGpropagate as an interference beam B_(ms) in the same direction, passthrough a spatial filter 38, and is received by a photoelectric element40. A photoelectric signal I_(ms) from the photoelectric element 40 hasthe waveform whose level sinusoidally changes at the same frequency asthe beat frequency 2Δf. The signal I_(ms) serves as the reference signalof the heterodyne scheme.

In the above arrangement, the reference grating SG is formed such that achromium layer is deposited on a glass plate and etched to alternatelyform transparent and light-shielding lines. For this reason, an almostideal grating, i.e., a grating having symmetrical amplitudetransmittances, almost free from at least asymmetry as in the gratingmark MG on the wafer W and the problem posed by the resist layer isformed. The pair of incident beams irradiated on the reference gratingSG may be incident beams having one of the wavelengths λ₁, λ₂, and λ₃ soas to obtain a sufficiently high precision. All the three 1st-orderdiffracted beams ±D₁₁, ±D₁₂, and ±D₁₃ included in the incident beams ±LFmay be simultaneously incident on the reference grating SG, therebyforming multi-color interference fringes as in the grating mark MG onthe wafer.

As described above, the multi-color interference fringes are formed onthe reference grating SG, and the interference beam Bms generated fromthis interference grating SG is separated in units of wavelengths andphotoelectrically detected. A reference signal corresponding to thewavelength λ₁, a reference signal corresponding to the wavelength λ₂,and a reference signal corresponding to the wavelength λ₃ can beindividually obtained, so that position measurement of the grating markMG can be performed in units of wavelengths. Even if the interferencefringes formed on the wafer W by three wavelength components are offset(phase offset) by a predetermined amount, the offset amount can bemeasured in advance. This technique will be described in detail later.

The wafer W shown in FIG. 24 is placed on a wafer stage WSTtwo-dimensionally moved within a plane (X-Y plane) perpendicular to theoptical axis of the objective lens 22. The two-dimensional movement onthe stage WST is performed by a drive source 42 including a drive motor.This driving may be based on a scheme for rotating a feed screw by amotor or a scheme for directly moving the stage itself by a linearmotor. In addition, the coordinate position of the stage WST issequentially measured by a laser interferometer 44. The measurementvalues of the laser interferometer 44 are used for feedback control forthe drive source 42. A fiducial mark plate FG is formed on part of thewafer stage WST. A reflection intensity grating (the pitch is equal tothat of the grating MG on the wafer) having a line-and-space patternwith a chromium layer on the surface of quartz is formed on the markplate FG. For this reason, unlike the phase grating such as the gratingmark MG formed by corrugations on the wafer W, the intensity grating ischaracterized in that no asymmetry is present and the diffractionefficiency does not depend on the wavelength of the illumination lightbeam (detection light beam), i.e., in that the amplitude reflectancedoes not have asymmetry. In addition, the reflectance of the chromiumlayer rarely changes in the wavelength band (generally 0.5 μm to 0.8 μm)of the position detection illumination light beam. For this reason, whenthe intensity grating on the fiducial mark plate FG is used, the changesin amplitudes and ratios between the amplitudes of the photoelectricsignals I_(m1), I_(m2), and I_(m3) obtained in units of wavelengths canbe accurately obtained.

In the arrangement shown in FIG. 24, semiconductor lasers are used asthe light sources. In this case, it is preferable that an astigmatismremoval shaping optical system (i.e., a plurality of inclinedplane-parallel glass members) be arranged between each semiconductorlaser (LS₂ and LS₃) and each of the dichroic mirrors DCM₄ and DCM₅, andthe diameters of the beam components of one synthesized beam LB₀ beequal to each other in units of wavelength components. In addition,it-is preferable that a beam shaping optical system for aligning thediameters of the wavelength components of the synthesized beam LB₀ bearranged.

For the sake of descriptive simplicity, the rotary radial grating plateRRG is used as the frequency shifter in FIG. 24. However, twoacousto-optic modulators (AOMs) may be used, or a first Zeeman laserlight source for oscillating a laser beam having a center frequency λ₁and a second Zeeman laser source for oscillating a laser beam having acenter frequency λ₂ may be used as light sources. In use of a Zeemanlaser, it generally oscillates two laser beams whose polarizationdirections are complementary, and a frequency difference of severalhundreds kHz to several MHz is given between these two laser beams. Thebeat frequency of an interference beam to be photoelectrically detectedis increased to a degree corresponding to the frequency difference. Thephotoelectric elements 36A, 36B, 36C, and 40 must be constituted by PINphotodiodes or photomultipliers having a high responsibility.

Various dichroic mirrors shown in FIG. 24 may be replaced withdispersion elements such as prisms. In this case, one prism has the samefunction as the set of two dichroic mirrors DCM₄ and DMC₅ or twodichroic mirrors 32 and 34.

An arrangement of a position detection/control circuit suitable for theapparatus shown in FIG. 24 will be described with reference to FIG. 25.In the heterodyne scheme in FIG. 25, while the interference beam BM isgenerated from the grating mark MG on the wafer W or the fiducial markplate FG, the signals I_(m1), I_(m2), I_(m3), and I_(ms) thephotoelectric elements 36A, 36B, 36C, and 40 have sinusoidal ACwaveforms shown in FIGS. 26A to 26D.

FIG. 26D shows a time change in intensity of the signal I_(ms) servingas a reference signal, while FIGS. 26A, 26B, and 26C show time changesin intensity of the signals I_(m1), I_(m2), and I_(m3) upon reception ofthe interference beam BM from the grating mark MG on the wafer W.Assuming the phase of the signal I_(ms) as a reference, the phase of thesignal I_(m1) is offset from the signal I_(ms) by −Δψ₁, the phase of thesignal I_(m2) is offset from the signal I_(ms) by −Δψ₂, and the phase ofthe signal I_(m3) is offset from the signal I_(ms) by −Δψ₃. Let E₁, E₂,and E₃ be the amplitudes (peak-to-peak values of the AC components) ofthe signals I_(m1), I_(m2), and I_(m3), respectively.

In the circuit block shown in FIG. 25, each of the signals I_(m1),I_(m2), I_(m3), and I_(ms) are input to an analog-to-digital conversion(A/D converter) circuit unit 50. The instantaneous intensity levels ofthe input signals are converted into digital values in response to aclock signal (pulse) C_(ps) from a sampling clock generator 52. Thefrequency of the clock signal C_(ps) is set much higher than the beatfrequencies of the signals I_(mn) (n=1, 2, 3) and I_(ms). The clocksignal C_(ps) is also sent to a waveform memory circuit unit 54 and isused to update the memory address in storing the digital values (data)from the A/D converter 50. In the waveform memory circuit unit 54, thefour waveform data shown in FIGS. 26A to 26D are digitally sampled forthe predetermined periods (e.g., 10 periods) of the signals I_(mn) andI_(ms). At this time, since the four signals I_(mn) and I_(ms) aresimultaneously sampled by the common clock signal C_(ps), the waveformdata in the waveform memory circuit unit 54 are not offset along thetime axis. Note that when the rotary radial grating plate RRG is used,the clock signal has a frequency of about ten and several kHz becausethe several kHz are the upper limit of the beat frequency. As inreference (E) (Japanese Patent Application Laid-open No. 6-82215), whenthe frequency shifter constituted by the two AOMs arranged in tandemwith each other is used, the beat frequency is determined by twice thedifference between the frequencies of the high-frequency modulationsignals applied to the respective AOMs and can be relatively freelydetermined.

The waveform data in the memory circuit unit 54 are loaded in adetection circuit unit 56 for phase differences Δψ_(n) (n=1, 2, 3) andposition offsets ΔX_(n) (n=1, 2, 3), so that the phase difference Δψ₁(Δψ₂ and Δψ₃) shown in FIGS. 26A (26B and 26C) is calculated by adigital arithmetic operation (Fourier integration). If the pitch Pmg ofthe grating mark MG of the wafer W and the pitch Pif of the interferencefringes formed thereon satisfy relation Pmg=2Pif, one period of eachwaveform in FIGS. 26A to 26D corresponds to Pmg/2. Phase measurement isgenerally performed within the range of ±180°. The calculation circuit56 converts the calculated phase differences Δψ₁, Δψ₂, and Δψ₃ intoposition offset amounts ΔX₁, ΔX₂, and ΔX₃ within the range of ±Pmg/4 inaccordance with equation (7). This offset amounts ΔXn represent offsetsof the grating mark with respect to the reference grating SG within therange of ±Pmg/4.

Assuming that the resolution of the phase difference measurement isgiven as about 0.2°, the resolution of the offset amount is about(0.2/180)Pmg/4. If the pitch Pmg is set to be 4 μm, a practical range of0.002 μm (2 nm) is obtained.

A signal amplitude and amplitude ratio detection circuit or circuit unit58 reads out the waveform data (FIGS. 26A to 26D) stored in the waveformmemory circuit unit 54 to detect the amplitude values E₁, E₂, and E₃ forthe respective waveforms. The detection circuit unit 58 prestoresamplitude values A₁, A₂, and A₃ of the photoelectric signals I_(m1),I_(m2), and I_(m3) obtained when the interference beam BM generated fromthe fiducial mark plate FG is received by the photoelectric elements36A, 36B, and 36C in advance.

The grating mark of the fiducial mark plate FG is moved below theobjective lens 22 prior to measurement of the grating mark on the waferW, and signals shown in FIGS. 26A to 26C are generated from thephotoelectric elements 36A, 36B, and 36C and stored in the waveformmemory unit 54. The amplitude values A₁, A₂, and A₃ are detected by theamplitude detection circuit 58 and stored. At this time, the stopposition of the stage WST which corresponds to detection of the fiducialmark plate FG is read by the laser interferometer 44, and positionoffset amounts ΔX_(b1), ΔX_(b2), and ΔX_(b3) of the respectivewavelengths are obtained by the offset amount detection circuit unit 56.These values can be utilized as data for determining a baseline.

The baseline here means a small mutual error component when the positionoffset amounts ΔX_(b1), ΔX_(b2), and ΔX_(b3) of the grating mark on themark plate FG, which are measured in units of wavelengths, are slightlydifferent from each other. When the interference fringes of therespective wavelengths which are formed by the beams of the wavelengthsλ₁, λ₂, and λ₃ on the fiducial mark plate FG strictly coincide with eachother in the incident system shown in FIG. 24, and the electricalresponse characteristics and the distortion characteristics sufficientlymatch each other, the values of the position offset amounts ΔX_(b1),ΔX_(b2), and ΔX_(b3) of the mark plate FG must be perfectly equal toeach other.

As a practical problem, however, when the resolution is about 2 nm, itis difficult to adjust the incident system and the detection system soas to match the position offset amounts ΔX_(b1), ΔX_(b2), and ΔX_(b3) inaccordance with the degree of resolution. For this reason, the mutualdifferences between the position offset amounts ΔX_(b1), ΔX_(b2), andΔX_(b3) measured by the mark plate FG are left as offsets (baselineerrors) unique to the alignment system shown in FIG. 24.

The baseline errors are determined by detecting the grating mark MG onthe wafer W and causing the position offset amounts ΔX₁, ΔX₂, and ΔX₃ ofthe respective wavelengths obtained by the detection circuit 56 by theposition offset amounts ΔX_(b1), ΔX_(b2), and ΔX_(b3) previouslydetermined. As an example, since the interference beam B_(ms) obtainedfrom the reference grating SG in the apparatus shown in FIG. 24 islimited to the wavelength λ₁, ΔX_(b2)−ΔX_(b1)=ΔX_(b21) andΔX_(b3)−ΔX_(b1)=ΔX_(b31) are calculated and stored with reference to themeasured position offset amount ΔX_(b1) of the fiducial mark plate FG.The value of the amount ΔX₂ is corrected and calculated so as to obtainΔX₂−ΔX₁=ΔX_(b21) with respect to the position offset amounts ΔX₁, ΔX₂,and ΔX₃ measured for the grating mark MG on the wafer W. Subsequently,the amount ΔX₃ is corrected and calculated so as to obtainΔX₃−ΔX₁=ΔX_(b31).

The interference beam B_(ms) obtained from the reference grating SG isset to include the respective wavelengths λ₁, λ₂, and λ₃. When theinterference beams of the respective wavelengths are to be individuallyphotoelectrically detected to obtain reference signals, the positionoffset amounts ΔX_(b1), ΔX_(b2), and ΔX_(b3) of the fiducial mark plateFG are obtained in units of reference signals (wavelengths). Themeasured position offset amounts ΔX₁, ΔX₂, and ΔX₃ of the grating markMG on the wafer are corrected and calculated as ΔX₁−ΔX_(b1),ΔX₂−ΔX_(b2), and ΔX₃−ΔX_(b3).

The amplitude ratio detection circuit unit 58 calculates ratios C₁, C₂,and C₃ of the amplitude values E₁, E₂, and E₃ obtained upon detection ofthe grating mark MG on the wafer W to the prestored amplitude values A₁,A₂, and A₃ as C₁=E₁/A₁, C₂=E₂/A₂, and C₃=E₃/A₃. The ratios C₁, C₂, andC₃ correspond to the weighting factors described in the embodiment shownin FIG. 21.

The data of the position offset amounts ΔX₁, ΔX₂, and ΔX₃ and the ratiosC₁, C₂, and C₃ are sent to a weighted means calculation circuit orcircuit unit 60. The circuit unit 60 calculates a weighted offset valueΔX of the grating mark MG as follows:

ΔX(C ₁ ·ΔX ₁ +C ₂ ·ΔX ₂ +C ₃ ·ΔX ₃)/(C ₁ +C ₂ +C ₃)

The resultant offset amount ΔX is an offset of the grating mark MG withrespect to the reference grating SG in the pitch direction. This data issupplied to a position controller (display controller) 62 and to a servocontrol circuit unit 64 when the wafer W is aligned (positioned) in realtime.

This servo control circuit unit 64 has two functions as described inFIG. 9. One function is to perform feedback control for the drive source42 until the offset amount ΔX reaches a predetermined value (directservo mode). To perform this function, the A/D converter 50, the memorycircuit unit 54, the offset amount detection circuit unit 56, and thecircuit unit 60 are repeatedly operated to calculate the offset amountΔX every very short period of time (e.g, several msec.) Note that thecalculations of the ratios C₁, C₂, and C₃ may be calculated once by theamplitude ratio detection circuit unit 58, or may be calculated everytime the offset amount ΔX is calculated. When the ratios C₁, C₂, and C₃are calculated every time, the values of the ratios C₁, C₂, and C₃slightly change every time the circuit unit 60 calculates the offsetamount ΔX, as a matter of course. When the ratios C₁, C₂, and C₃ are tobe calculated once or a plurality of the number of times, the calculatedratio values are used during subsequent detection of the same gratingmark MG.

The other function of the servo control circuit unit 64 is a function ofmoving the wafer stage WST on the basis of a measurement value from thelaser interferometer 44 (interferometer servo mode). This function isused to position the grating of the fiducial mark plate FG on the stageWST or the grating mark MG on the wafer W immediately below theobjective lens 22, or positioning an arbitrary point on the wafer Wimmediately below the objective lens 22 with reference to the detectedposition of the grating mark MG. In the interferometer servo mode,target position information of the wafer stage WST is output from theposition controller 62 to the servo control circuit unit 64. The controlcircuit unit 64 performs feedback control of the drive source 42 suchthat a difference between the target position and the current positionof the stage WST which is read by the laser interferometer 44 fallswithin a predetermined allowable range (e.g., ±0.04 μm).

The direct servo mode can be set following the interferometer servo modein the same manner as in FIG. 9.

The position controller (display controller) 62 also has a function ofdisplaying the coordinate position of the grating mark MG and theobtained offset amount ΔX in addition to the function of designatingswitching between the servo modes described above. The positioncontroller (display controller) 62 may often store and hold the valuesof the ratios C₁, C₂, and C₃ serving as the weighting factors upondetection of the grating mark MG. Assume that a large number ofidentical grating marks MG are formed on the wafer W, and that thepositions of these marks MG are to be sequentially detected. In thiscase, when the ratios C₁, C₂, and C₃ are sequentially stored, a specificmark MG on the wafer W which causes asymmetry and nonuniformity of theresist layer can be checked. Portions on the wafer W where the weightingfactor (ratios C₁, C₂, and C₃) greatly change may be graphicallydisplayed. At this time, when the changes in weighting factors areobtained by mounting a wafer prior to coating of the resist layer afterthe chemical process such as diffusion and etching in the apparatus ofFIG. 24, the influences of the chemical process on the wafer surface canalso be indirectly checked. In addition, when the resist layer is formedon the wafer to measure changes in weighting factors, and these changesare compared with the changes in weighting factors prior to coating ofthe resist layer, the influence of the resist layer can also beindirectly checked.

In the 10th embodiment, the fiducial mark plate FG is placed on thestage WST and used to obtain the rates of changes in signal amplitudesof the respective wavelengths, i.e., the ratios C₁, C₂, and C₃. Thephotoelectric elements DT₁ and DT₂ for directly detecting the lightintensities of the incident beams LB₁ and LB₂ as in the eighthembodiment (FIG. 21) need not be arranged. To the contrary, when thefiducial mark plate FG serving as the reference in the eighth (ninth)embodiment is arranged in tandem with the grating MG, the ratios C₁ andC₂ can be detected without arranging the photoelectric elements DT₁ andDT₂.

FIG. 27 shows the arrangement of a signal processing circuit of the 11thembodiment. In this embodiment, a wavelength selection filter 24 shownin FIG. 24 is omitted. An interference beam B_(ms) from a referencegrating SG is separated into beams B_(ms1), B_(ms2), and B_(ms3) forrespective wavelengths λ₁, λ₂, and λ₃ by a dichroic mirror and the like.The beams B_(ms1), B_(ms2), and B_(ms3) are photoelectrically detectedby three photoelectric elements 40A, 40B, and 40C, respectively. Phasedifferences between signals I_(m1), I_(m2), and I_(m3) fromphotoelectric elements 36A, 36B, and 36C and reference signals I_(ms1),I_(ms2) and I_(ms3) from the photoelectric elements 40A, 40B, and 40Care detected. More specifically, a phase difference Δψ₁ between themeasurement signal I_(m1) and the reference signal I_(ms1) is obtainedto obtain a position offset (ΔX₁) of the grating mark MG using anincident beam having the wavelength λ₁.

With this arrangement, the number of signals for receiving waveform datais large. As shown in FIG. 27, three waveform sampling circuits (eachhaving the functions of an A/D converter 50, a clock generator 52, and awaveform memory circuit 54) 80A, 80B, and 80C are arranged incorrespondence with the wavelengths. The internal arrangements of thecircuits 80A, 80B, and 80C are identical to each other, and only thedetailed internal arrangement of the circuit 80A is illustrated in FIG.27. A detailed description of the remaining circuits 80B and 80C will beomitted.

In this embodiment, as shown in the circuit 80A, the signal I_(m1) fromthe photoelectric element 36A which receives the measurementinterference beam B_(m1) and the signal I_(ms1) from the photoelectricelement 40A which receives the reference interference beam B_(ms1) areinput to sample/hold (S/H) circuits 800 and 802, respectively. Thesignal levels from the S/H circuits 800 and 802 are input to ananalog-to-digital converter (ADC) 806 through an analog multiplexer 804.

The digital value output from the ADC 806 is written at an accessedaddress of a random access memory (RAM) 808. An address value for theRAM 808 is generated by an address counter 810, and the address value isincremented (decremented) in response to a clock signal C_(ps). Notethat the address counter 810 has a special function to supply the clocksignal C_(ps) as a flag to a specific one of the upper bits of theaddress counter. Therefore, the address space of the RAM 808 is dividedinto two pages. While the clock signal C_(ps) is set at logic “0”, theaddress space of the first page is accessed. While the clock signalC_(ps) is set at logic “1”, the address space of the second page isaccessed.

The clock signal C_(ps) is also supplied to the S/H circuit 802. Theclock signal C_(ps) is further supplied to a timing circuit 814 and usedas a digital conversion timing signal and a write timing signal (pulse)for the data to be written in the RAM 808.

When the clock signal C_(ps) is set at “1”, the S/H circuit 802 is setin a hold state, and the level of the signal I_(m1) from thephotoelectric element 36A is supplied to the ADC 806 through the analogmultiplexer 804. A digital value corresponding to this level is storedat one address position in the address space of the second page of theRAM 808. When the clock signal C_(ps) is set at “0”, the S/H circuit 802is set at the hold state. The level of the signal I_(ms1) from thephotoelectric element 40 is supplied to the ADC 806 through themultiplexer 804. A digital value corresponding to this level is storedat one address position of the address space of the first page of theRAM 808.

The above operation is repeated a predetermined number of periods (e.g.,10 periods or more) of the signal I_(m1) (or I_(ms1)) at high speed. Thewaveform data of the reference signal I_(ms1) is stored in the firstpage of the RAM 808, while the waveform data of the measurement signalI_(m1) is stored in the second page. The pair of waveform data thusstored in the RAM 808 are read out onto a data bus DBS of amicroprocessor in response to address values set in the address counter810. The microprocessor processes the waveform data in accordance with aprogram for achieving the same function as the detection circuits 56 and58 in FIG. 6, thereby obtaining the position offset amounts ΔX₁, ΔX₂,and ΔX₃.

The arrangement and operation of the sampling circuit 80A are the sameas those of the sampling circuits 80B and 80C. The circuit 80Btemporarily stores the waveform data between the measurement signalI_(m2) and the reference signal I_(ms2), and similarly the circuit 80Ctemporarily stores the waveform data between the measurement signalI_(m3) and the reference signal I_(ms3).

In the 11th embodiment, since the A/D converters are not arranged forboth the measurement signal I_(mn) and the reference signal I_(msn), theoperation for simultaneously sampling these signals on the microsecondorder is difficult. However, in practice, when the beat frequency of theinterference beam is several tens kHz or less, simultaneity on themicrosecond order is not so required. It is more advantageous to halfthe number of A/D converters and the like so as to simplify the circuitarrangement, thereby reducing the hardware cost.

When a fiducial mark plate FG having a chromium surface having a knownreflectance is fixed on a wafer stage WST, this mark plate FG can beused for measuring various baseline amounts and focus states, aspreviously described. The baseline amount basically means an actuallymeasured value for determining the relative positional relationshipbetween the projection point of the center of a mask (reticle) mountedin the projection exposure apparatus and the detection center point ineach of the various alignment systems.

When the position detection apparatus shown in FIG. 24 is applied toeach alignment system of the projection exposure apparatus shown in FIG.15, detection center points R_(f1), R_(f2), R_(f3), and R_(f4) aredefined by the reference grating SG. In a reticle alignment system RA,when a reticle alignment mark (grating pattern) RM in the peripheralportion of the reticle R and the corresponding grating mark on thefiducial mark plate FG are irradiated with an illumination light beamhaving the same wavelength as that of the illumination light beam forprojecting and exposing the pattern PR, and the reticle stage RST isfinely moved so that both the marks have a predetermined positionalrelationship, the detection center point R_(f1) need not be used.

This also applies to an alignment system TTRA. When the correspondingmark on the fiducial mark plate FG or the mark on the wafer, and adie-by-die (D/D) alignment mark formed in the peripheral portion of thepattern PR of the reticle R are imaged, and a position offset betweenthese two mark images is detected, the detection center point R_(f2)need not be defined.

The baseline amounts indicate the X-Y positional relationship betweenthe projection point (substantially coinciding with an optical axis AX)of a center CCr of a reticle R on the wafer and the projection points ofthe detection center points R_(f1), R_(f2), R_(f3), and R_(f4) on thewafer. This positional relationship can be obtained by causing thealignment systems RA and TTRA and alignment systems TTLA and OFA todetect the position offset amounts between the corresponding marks onthe fiducial mark plate FG and the projection points of the detectioncenter points R_(f1) to R_(f4), and at the same time causing a laserinterferometer 44 (see FIG. 24) to detect the corresponding coordinateposition of the wafer stage WST.

When the position detection apparatus of the heterodyne scheme shown inFIG. 24 is incorporated in each alignment system, the grating of thefiducial mark plate FG is detected during baseline measurementoperation. For this reason, the amplitude levels A₁. A₂, and A₃ of thesignals I_(m1), I_(m2), and I_(m3) from the photoelectric elements 36A,36B, and 36C in FIG. 24 can be stored in the circuit unit 58 in FIG. 25.Note that a pupil plane EP of a projection optical system PL shown inFIG. 15 is identical to a Fourier transform plane EP shown in FIG. 21.The optical axes of objective lenses arranged in the alignment systemsRA, TTRA, and TTLA for detecting objects (the mark on the wafer W andthe mark of the fiducial mark plate FG) on the wafer stage WST throughthe projection optical system PL are substantially parallel to theoptical axis AX on the wafer stage WST side. When the reticle side ofthe projection optical system PL as well as its wafer side is settelecentric (FIG. 15), the optical axes of the objective lenses of thealignment systems are set parallel to the optical axis AX of theprojection optical system PL. The extended lines of the optical axes ofthe objective lenses pass the center (a portion through which theoptical axis XX passes) of the pupil plane EP-of the projection opticalsystem PL.

The effective radius of the pupil plane EP corresponds to the numericalaperture (NA) which determines the resolving power (minimum resolutionline width) of the projection lens PL. A projection lens having NA=about0.5 to 0.7 is being developed at present.

FIG. 28 shows the main part of the alignment system TTLA of all thealignment systems shown in FIG. 15. The pair of incident beams ±LF(corresponding to the beam +LF and the beam −LF in FIG. 24) fordetecting the grating mark MG on the wafer or the fiducial mark plate FGare incident on the projection lens PL through a correction opticalsystem CG, a polarizing beam splitter PBS (functionally corresponding tothe half mirror 20 in FIG. 24), an objective lens OBJ (corresponding tothe objective lens 22 in FIG. 24), and two mirrors MR. In this case, aplane FC conjugate to the surface of the wafer W is formed between thetwo mirrors MR. The pair of beams ±LF cross on this plane FC. The beams±LF are relayed by the projection lens PL and also cross on the wafer,thereby irradiating the grating mark MG.

The interference beam BM from the grating mark MG passes almost thecenter of the pupil plane PE of the projection lens PL, is incident on adichroic mirror DCM (corresponding to a dichroic mirror 32 in FIG. 24)through the mirrors MR, the objective lens OBJ, and the beam splitter20, and then wavelength-divided. If the incident beams ±LF have thewavelengths λ₁ and λ₂, the dichroic mirror DCM guides the interferencebeam B_(m1) having the wavelength λ₁ to the photoelectric element 36Aand the interference beam B_(m2) having the wavelength λ₂ to thephotoelectric element 36B.

In this alignment system TTLA, when the incident bemas ±LF include aplurality of wavelength components (these components are separated fromeach other by about 30 nm to 40 nm), the crossing region of the beams±LF irradiated on the wafer is shifted in the Z direction or the X and Ydirections due to the influence of the chromatic aberration (on-axialmagnification factor) or the influence of the chromatic aberration ofthe objective lens OBJ. The correction optical system CG for correctingthe errors generated in accordance with the chromatic aberrations isarranged in the optical path of the incident beams ±LF. This correctionoptical system CG comprises a convex lens, a concave lens, a combinationthereof, or a plane-parallel glass member. Alternatively, the correctionoptical system CG may be constituted by the adjustment optical systems14, 16, and 18 shown in FIG. 24.

In the alignment system TTRA in FIG. 15, a D/D alignment mark DDM on thereticle R serves as a diffraction grating. When a relative positionoffset between the mark DDM and the corresponding grating mark MG on thewafer W is to be detected in accordance with the heterodyne scheme shownin FIG. 24, a transparent plane-parallel correction plate PGP isarranged on a pupil plane EP of a projection lens PL, as shown in FIGS.17A and 17B. Phase diffraction gratings PG1, PG2, and PG3 are arrangedat only positions where the incident beams (±LF) and the interferencebeam (BM) pass on the correction plate PGP, thereby reducing theinfluences of the chromatic aberration of on-axial and the chromaticaberration of magnification.

The 12th embodiment of the present invention will be described below. Inthis embodiment, using the arrangement shown in FIG. 24 as a base, aninterference beam of 0th- and 2nd-order diffracted light components froma grating mark are detected in addition to the interference beam of the±1st-order diffracted light components from the grating mark, asdescribed in FIGS. 4, 5A, and 5B. In a system wherein the interferencebeam of the 0th- and 2nd-diffracted light components isphotoelectrically converted by a single photoelectric element to detecta position offset of the grating mark using this photoelectric signal,when an interference beam (multi-wavelength beam) of the 0th- and2nd-order diffracted light components upon obtaining a multi-wavelengthincident beam for illuminating a grating mark is received by the singlephotoelectric element, it is difficult to properly detect the positionoffset. The main reason for this can be easily understood from FIGS. 29Ato 29D, when the waveforms of photoelectric signals IK02 ₁, IK02 ₂, andIK02 ₃ obtained upon photoelectric detection of the interference beamsof the 0th- and 2nd-order diffracted light components, e.g., threewavelength components λ₁, λ₂, and λ₃ are observed. That is, as shown inFIGS. 29A to 29D, the phase differences between the three photoelectricsignals IK02 _(n) (n=1, 2, 3) are larger than those between thephotoelectric signals I_(mn) (FIG. 7) of the interference beams of the±1st-order diffracted light components. For this reason, when changes inintensities of the wavelengths having the large phase differences arereceived by the single photoelectric element, the amplitudes (ACamplitude components) of the photoelectric signals become very small bythe canceling effect of the wavelength intensities. Note that theinterference beams of the 0th- and 2nd-order diffracted light componentsare generated on the two sides of an interference beam BM of the1st-order diffracted light components ±D_(1n) at symmetrical angles.

FIGS. 29A, 29B, and 29C show the heterodyne waveforms of thephotoelectric signals IK02 ₁, IK02 ₂, and IK02 ₃ when the interferencebeam of all the interference beams of the 0th- and 2nd-order diffractedlight components which appears on the left side of the interference beamBM of the ±1st-order diffracted light components is detected for thethree wavelengths λ₁, λ₂, and λ₃. FIG. 29D shows the waveform of aphotoelectric signal I_(ms) serving as the reference signal as in FIG.26D.

FIGS. 30A, 30B, and 30C show the heterodyne waveforms of photoelectricsignals IK20 ₁, IK20 ₂, and IK20 ₃ when the interference beam of all theinterference beams of the 0th- and 2nd-order diffracted light componentswhich appears on the right side of the interference beam BM of the±1st-order diffracted light components is detected for the threewavelengths λ₁, λ₂, and λ₃. FIG. 30D shows the waveform of aphotoelectric signal I_(ms) serving as the reference signal as in FIG.29D. As shown in FIGS. 29A, 29B, and 29C and FIGS. 30A, 30B, and 30C,phase offsets Δβ₀₁, Δβ₀₂, Δβ₀₃, Δβ₂₁, Δβ₂₂, and Δβ₂₃ of the signals IK02_(n) and IK20 _(n) (n=1, 2, 3) have a strong wavelength dependence andgreatly vary. At the same wavelength, the signals IK02 _(n) and IK20_(n) tend to have components of opposite directions.

The arrangement of this embodiment will be described with reference toFIG. 31. FIG. 31 shows part of the arrangement of FIG. 24 and is amodification of a photoelectric detection system for detecting variousinterference beams from a grating mark MG. The same reference numeralsas in FIG. 24 denote the parts having the same functions in FIG. 31.Referring to FIG. 31, an incident system 100 includes light sources LS₁,LS₂, and LS₃, a mirror MR, dichroic mirrors DCM₄ and DCM₅, a radialgrating plate RRG serving as a frequency shifter, a lens 10, a spatialfilter 12, and adjacent optical systems 14, 16, and 18. The incidentsystem 100 emits a pair of incident beams +LF and −LF. The incidentbeams ±LF including the wavelengths λ₁, λ₂, and λ₃ are partiallyreflected by a half mirror 20 and incident on an objective lens 22. Theremaining part of the beams is incident on a reference light receptionsystem 110. The reference light reception system 110 comprises awavelength selection filter 24, a lens 26, a reference grating SG, and aspatial filter 38 in FIG. 24. The reference light reception system 110guides a reference light beam B_(ms) to a photoelectric element 40. Whena grating MG on a wafer W is irradiated with the incident beams ±LFthrough the objective lens 22, an interference beam BM of the ±1st-orderdiffracted light components is vertically generated from the grating MG.At the same time, interference beams of the 0th- and 2nd-orderdiffracted light components are generated in a direction opposite to thetraveling direction of each incident beam. The interference beam of the0th- and 2nd-diffracted light components is directed to dichroic mirrors32 and 34 through the objective lens 22 and the half mirror 20 anddivided into wavelength components. The dichroic mirror 32 reflects mostof the interference beams (two beams) of the 0th- and 2nd-orderdiffracted light components which have the wavelength λ₁, and thereflected beams are received by photoelectric elements 36A₁ and 36A₂. Aninterference beam B_(m1) of the ±1st-order diffracted light componentshaving the wavelength λ₁ is reflected by the dichroic mirror 32, and thereflected beam is received by a photoelectric element 36A.

Interference beams B_(m2) and B_(m3) of the 0th- and −2nd-orderdiffracted light components and of the ±1st-order diffracted lightcomponents, which respectively have the wavelengths λ₂ and λ₃ and havepassed through the dichroic mirror 32, are separated into wavelengthcomponents by the dichroic mirror 34. The interference beams (two beams)of the 0th- and −2nd-order diffracted light components, which have thewavelength λ₂, are respectively received by photoelectric elements 36B₁and 36B₂. The interference beam B_(m2) of the ±1st-order diffractedlight components is received by a photoelectric element 36B. Theinterference beams (two beams) of the 0th- and −2nd-order diffractedlight components, which have the wavelength λ₃ and have passed throughthe dichroic mirror 34, are respectively received by photoelectricelements 36C₁ and 36C₂. The interference beam B_(m3) of the ±1st-orderdiffracted light components is received by a photoelectric element 36C.

As can be apparent from the above arrangement, in this embodiment, asignal processing circuit is required to obtain the phase differencesbetween the photoelectric signals from the photoelectric elements 36A,36A₁, 36A₂, 36B, 36B₁, 36B₂, 36C, 36C₁, and 36C₂ using the photoelectricsignal I_(ms) as the reference signal from the photoelectric element 40.The simplest circuit arrangement of the signal processing circuit isshown in FIG. 32.

FIG. 32 is an improvement of part of the processing circuit shown inFIG. 25. The circuit shown in FIG. 32 is different from that in FIG. 25in that an analog multiplexer 120 as a hardware element is added totime-serially select each photoelectric signal except for the referencesignal I_(ms) input to an A/D converter 50 in FIG. 25. The analogmultiplexer 120 includes 3-input 1-output switches SS₁, SS₂, and SS₃.The switches SS₁, SS₂, and SS₃ are operated in response to an externalswitching signal SN.

The switch SS₁ selects one of the three photoelectric signals I_(m1),IK02 ₁, and IK20 ₁ obtained upon reception of the interference beams ofthe wavelength λ₁. The switch SS₂ selects one of the three photoelectricsignals I_(m2), IK0 ₂₂, and IK20 ₂ obtained upon reception of theinterference beams of the wavelength λ₂. The switch SS₃ selects one ofthe three photoelectric signals I_(m3), IK02 ₃, and IK20 ₃ obtained uponreception of the interference beams of the wavelength λ₃. In thisembodiment, since the switches SS₁ to SS₃ are interlocked, the threemeasurement signals (photoelectric signals) simultaneously input to theA/D converter 50 are given as signals detected in the same diffractedstate. More specifically, when the switches SS₁ to SS₃ are switched tothe intermediate positions, the same state as in FIG. 25 is set. Thesignals I_(m1) to I_(m3) obtained by photoelectrically detecting theinterference beam of the ±1st-order diffracted light components aresupplied to the A/D converter 50. When the three switches SS₁ to SS₃ areswitched to the illustrated positions in FIG. 32, the signals IK02 ₁,IK02 ₂, and IK02 ₃ obtained by photoelectrically detecting, in units ofwavelengths, the interference beam of the 0th- and -2nd-order diffractedlight components, which appears on the left side of the interferencebeam BM of the ±1st-order diffracted light components, are supplied tothe A/D converter 50. When the switches SS₁ to SS₃ are switched to theright positions, the photoelectric signals IK20 ₁, IK20 ₂, and IK20 ₃are supplied to the A/D converter 50.

An amplitude detection and amplitude ratio detection circuit 58 shown inFIG. 25 is changed to output ratio data C_(n1), C_(n2), and C_(n3) (n=1,2, 3 corresponding to the wavelengths) grouped in units of interferencebeams having different diffracted states in FIG. 32. Of these ratiodata, ratio data C_(n1), (n=1, 2, 3) are identical to ratios C₁, C₂, andC₃ in FIG. 25. The ratio data C_(n2) (n=1, 2, 3) are ratios obtainedfrom the photoelectric signals IK02 _(n) (n=1, 2, 3) in units ofwavelengths. The ratio data C_(n3) (n=1, 2, 3) are ratios obtained fromthe photoelectric signals IK20 _(n) (n=1, 2, 3) in units of wavelengths.

A phase difference and position offset detection circuit 56 shown inFIG. 25 is modified to output offset amounts ΔX_(n1), ΔX_(n2), andΔX_(n3) (n=1, 2, 3) grouped in units of interference beams havingdifferent diffracted states in FIG. 32. Of these offset amounts, theoffset amounts ΔX_(n1) (n=1, 2, 3) are equal to offset amounts ΔX₁, ΔX₂,and ΔX₃ in FIG. 6. The offset amounts ΔX_(n2) (n=1, 2, 3) are obtainedfrom the photoelectric signals IK02 _(n) (n=1, 2, 3) in units ofwavelengths. The offset amounts ΔX_(n3) (n=1, 2, 3) are obtained fromthe photoelectric signals IK20 _(n) (n=1, 2, 3) in units of wavelengths.Note that this detection circuit 56 calculates, as intermediate values,values corresponding to the phase differences Δβ_(0n) and Δβ_(2n) (n=1,2, 3) described in FIGS. 29A to 29D and FIGS. 30A to 30D.

A weighted mean calculation circuit unit 60 is modified into a selectiveweighted mean calculation circuit in FIG. 32. This circuit has the firstarithmetic mode for calculating a final position offset amount ΔX on thebasis of the photoelectric detection results of the interference beam ofthe ±1st-order diffracted light components as in FIG. 25, the secondarithmetic mode for calculating the final offset amount ΔX on the basisof the photoelectric detection results of the interference beams of the0th- and −2nd-order diffracted light components, and the thirdarithmetic mode for calculating the final offset amount ΔX on the basisof the detection results of all the interference beams. These threearithmetic operation modes can be arbitrarily selected by the operator.When the third arithmetic mode is designated, a few additionalarithmetic algorithms can be selected. Such mode designation andalgorithm designation will be described in detail later on.

In this embodiment, a wafer stage WST is positioned to irradiateincident beams ±LF from the objective lens 22 onto the grating mark of afiducial mark plate FG on the wafer stage WST. The switching signal SNis supplied to the analog multiplexer 120 to set the switches SS₁ to SS₃to the positions illustrated in FIG. 32. The signals IK02 _(n) (n=1, 2,3) of the photoelectric signals obtained upon photoelectric detection ofthe interference beams of the 0th- and −2n-order diffracted lightcomponents generated by the grating mark of the fiducial mark plate FGare digitally sampled by the A/D converter 50. The waveforms of thesignals IK02 _(n) are temporarily stored in a waveform memory circuit54.

An amplitude detection circuit 58 analyzes the waveform data stored inthe memory circuit 54 and calculates and stores the amplitude values(peak-to-peak values) of the signals IK02 _(n) as values J02 _(n) (n=1,2, 3).

Then the switches SS₁ to SS₃ are switched to the right positions in FIG.32. The signal IK20 _(n) (n=1, 2, 3) of the photoelectric signalsobtained upon photoelectric detection of the interference beams of the0th- and −2nd-order diffracted light components generated from thegrating mark of the fiducial mark plate FG are digitally sampled by theA/D converter 50. The waveforms of the signals IK20 _(n) are temporarilystored in the memory circuit 54. In this case, when the storage capacityof the memory circuit 54 is not so large, the previously stored waveformdata of the signals IK02 _(n) are erased, and the waveform data of thesignals IK20 _(n) are overwritten- The-waveform data in the memorycircuit 54 are then analyzed by the amplitude detection circuit 58, andthe amplitude values (peak-to-peak values) of the signals IK20 _(n)(n=1, 2, 3) are calculated and stored as values J20 _(n) (n=1, 2, 3).

Finally, the switches SS₁ to SS₃ are switched to the intermediatepositions. Waveform data of the signals I_(mn) (n=1, 2, 3) obtained byphotoelectrically detecting the interference beam of the ±1st-orderdiffracted light components generated from the grating mark of thefiducial mark plate FG are similarly stored in the memory circuit 54.Amplitude values J11 _(n) (n=1, 2, 3) are calculated and stored by theamplitude detection circuit 58.

When the preliminary operation is thus completed, the wafer W to beactually positioned and aligned is placed on the stage WST. The stageWST is positioned to irradiate the incident beams ±LF from the objectivelens 22 onto the grating mark MG on the wafer W.

In the same manner as in detection of the grating mark of the fiducialmark plate FG, the switches SS₁ to SS₃ of the multiplexer 120 aresequentially switched to store the waveform data of the respectivesignals in the memory circuit 54, and the amplitude values of thesignals I_(mn), IK02 _(n), and IK20 _(n) (n=1, 2, 3) obtained byphotoelectrically detecting the interference beams generated from thegrating mark MG on the wafer are calculated as E_(n) (FIGS. 26A to 26D)and E02 _(n) and E20 _(n) (FIGS. 29A to 29D and FIGS. 30A to 30D).

At the time of switching the switches SS₁ to SS₃ to store one of thesets of signals I_(mn), IK02 _(n), and IK20 _(n), the waveform of thereference signal I_(ms) is stored in the memory circuit 54 along thesame time axis. Prior to switching of the switches SS₁ to SS₃, thestored waveform data of all the waveform data of the signals I_(mn),IK02 _(n), and IK20 _(n) are analyzed by the phase difference and phaseoffset amount detection circuit 56. The circuit 56 sequentiallycalculates a corresponding one of phases Δψ_(n), Δβ_(0n), and Δβ_(2n)and a corresponding one of the position offset amounts ΔX_(n1), ΔX_(n2),and ΔX_(n3) (n=1, 2, 3).

When the amplitude values and the position offset amounts of therespective wavelengths are obtained in units of detection light beamshaving different diffracted states (in units of interference beams), theamplitude ratio detection circuit 58 performs the following arithmeticoperations: $\begin{matrix}\left. \quad \begin{matrix}{C_{11} = {E_{1}\text{/}{J11}_{1}}} \\{C_{21} = {E_{2}\text{/}{J11}_{2}}} \\{C_{31} = {E_{3}\text{/}{J11}_{3}}}\end{matrix}\quad \right\} & (7) \\\left. \begin{matrix}{C_{12} = {{E02}_{1}\text{/}{J02}_{1}}} \\{C_{22} = {{E02}_{2}\text{/}{J02}_{2}}} \\{C_{32} = {{E02}_{3}\text{/}{J02}_{3}}}\end{matrix}\quad \right\} & (8) \\\left. \begin{matrix}{C_{13} = {{E20}_{1}\text{/}{J20}_{1}}} \\{C_{23} = {{E20}_{2}\text{/}{J20}_{2}}} \\{C_{33} = {{E20}_{3}\text{/}{J20}_{3}}}\end{matrix}\quad \right\} & (9)\end{matrix}$

The most probable offset amount (deviation) ΔX is calculated by theweighted mean calculation circuit 60. In the first arithmetic mode usingonly the interference beam BM of the ±1st-order diffracted lightcomponents, the offset amount ΔX is calculated as follows in the samemanner as in FIG. 25:

ΔX=(C ₁₁ ·ΔX ₁₁ +C ₂₁ ·ΔX ₂₁ +C ₃₁ ·ΔX ₃₁)/(C ₁₁ +C ₂₁ +C ₃₁)

On the other hand, in the second arithmetic mode using only theinterference beam of the 0th- and −2nd-order diffracted lightcomponents, an algorithm is employed to calculate the position offsetamount of each wavelength in accordance with an average phase differencebetween phase differences Δβ_(0n), obtained upon detection of theinterference beam of the 0th- and −2nd-order diffracted lightcomponents, which appears on the left side of the interference beam BMof the ±+1st-order diffracted light components and phase differencesΔβ_(2n) obtained upon detection of the interference beam of the 0th- and−2nd-order diffracted light components, which appears on the right sideof the interference beam BM of the ±1st-order diffracted lightcomponents. This phase difference average is not an average for reducingso-called random components to improve precision, but an average whichmust be obtained in principle in position detection using theinterference beams of the 0th- and ±2nd-order diffracted lightcomponents.

Based on this algorithm of this embodiment, the weighted meancalculation circuit 60 calculates average values ΔXA_(n) (n=1, 2, 3) ofthe respective wavelengths between the position offset amounts ΔX_(n2)(n=1, 2, 3) obtained from the signals IK02 _(n) and the position offsetamounts ΔX_(n3) (n=1, 2, 3) obtained from the signals IK20 _(n) (n=1, 2,3) as follows:

ΔXA ₁=(ΔX ₁₂ +ΔX ₁₃)/2

ΔXA ₂=(ΔX ₂₂ +ΔX ₂₃)/2

ΔXA ₃=(ΔX ₃₂ +ΔX ₃₃)/2

The weighted mean calculation circuit 60 also calculates average valuesCA_(n) (n=1, 2, 3) of the amplitude ratios C_(n2) and C_(n3) of the 0th-and −2nd-order diffracted light components of the respective wavelengthsobtained in the amplitude ratio detection circuit 58 as follows:

CA ₁=(C ₁₂ +C ₁₃)/2

CA ₂=(C ₂₂ +C ₂₃)/2

CA ₃=(C ₃₂ +C ₃₃)/2

The weighted mean calculation circuit 60 then calculates the weightedmean value of the average position offset amounts ΔXA_(n) using theaverage ratios CA_(n) of the respective wavelength components asweighting factors, thereby calculating the most probable offset amountΔX as follows:

ΔX=(CA ₁ ·ΔXA ₁ +CA ₂ ·ΔXA ₂ +CA ₃·ΔXA₃)/(CA ₁ +CA ₂ +CA ₃)

By the above calculation, position detection and position offsetdetection of the grating mark in the second arithmetic mode can beachieved.

In the third arithmetic mode, the operator can arbitrarily set one ofthe first algorithm for simply averaging the position offset amountcalculated in the first arithmetic mode and the position offset amountcalculated in the second arithmetic mode, and the second algorithm forcalculating the weighted mean of these two position offset amounts. LetΔXM₁ be the position offset amount finally calculated in the firstarithmetic mode (i.e., the mode using the detection results of theinterference beam of the ±1st-order diffracted light components) andΔXM₂ be the position offset amount finally calculated in the secondarithmetic mode. In this case, the position offset amount determined bythe first algorithm is calculated as (ΔXM₁+ΔXM₂)/2.

On the other hand, in the second algorithm, the weighted mean value ofthe offset amount ΔXM₁ calculated in the first arithmetic mode and theoffset amount ΔXM₂ calculated in the second arithmetic mode arecalculated with predetermined weighting factors Q₁ and Q₂. As anexample, the weighting factor Q₁ is caused to correspond to the sum ofthe amplitude values E₁, E₂, and E₃ (see FIGS. 26A to 26D) of thesignals Imn (n=1, 2, 3) obtained upon photoelectric detection of theinterference beam BM of the ±1st-order diffracted light components, andthe weighting factor Q₂ is caused to correspond to the sum of theaverage amplitude values (E02 ₁+E20 ₁)/2, (EO2 ₂+E20 ₂)/2, and (EO2₃+E20 ₃)/2 of the signals IK02 _(n) and IK20 _(n) (n=1, 2, 3) obtainedfor photoelectrically detecting the interference beam of the 0th- and−2nd-order diffracted light components in units of wavelengths.Therefore, the offset amount ΔX of the grating mark MG is determined bythe following calculation in the second algorithm:

 ΔX=(Q ₁ ·ΔXM ₁ +Q ₂ ·ΔXM ₂)/(Q ₁ +Q ₂)

In principle, diffracted light components of higher order have lowerintensities. For this reason, the light intensity amplitudes(corresponding to E_(n)) of the interference beam BM of the ±1st-orderdiffracted light components are much larger than those (corresponding toE02 _(n) and E20 _(n)) of the ,interference beam of the 0th- and−2nd-order diffracted light components. When the weighting factors Q₁and Q₂ are simply determined by the sums of the amplitudes of thesignals I_(mn), IK02 _(n), and IK20 _(n), the weighting factor Q₁ isusually larger than the weighting factor Q₂. Therefore, the calculatedvalue with respect to the weighting factor Q₂ is preferably corrected toincrease by, e.g., a predetermined ratio (e.g., 10% to 30%).

The 13th embodiment of the present invention will be described withreference to FIG. 33. In this embodiment, the structure of a fiducialmark plate FG on a wafer stage WST shown in FIG. 24 is replaced with atransmission grating (i.e., a grating whose amplitude transmittance doesnot have asymmetry). An interference beam transmitted through thisgrating is photoelectrically detected to obtain a denominator (referencevalue) used in causing a detection circuit 58 to calculate the amplituderatios of photoelectric signals I_(mn), IK02 _(n), and IK20 _(n).

FIG. 33 shows the partial section of the wafer stage WST. When incidentbeams tLF (in this case, the beams have two wavelengths λ₁ and λ₂) areirradiated on the grating of the fiducial mark plate FG, 0th-, ±1st- and±2nd-order diffracted light components are generated from the gratingtoward the interior of the stage. These diffracted light components aredeflected at a right angle by a mirror MR and incident on a lens systemG5 having a Fourier transform function. One of the components of thewavelengths λ₁ and λ₂ is selected by a wavelength selection filter 24having an automatic exchanging function, and result beams becomeinterference beams B_(mrn), ±B_(1r), and ±B_(2r) which are then incidenton photoelectric elements DTR. The wavelength selection filter 24 isarranged such that a filter for transmitting the wavelength λ₁ andshielding the wavelength λ₂ and a filter having characteristics oppositeto those of the above filter can be alternatively inserted into orretracted from the optical path. When the filter for selecting thewavelength λ₁ is used, the interference beams ±B_(2r) of the 0th- and−2nd-order diffracted light components having the wavelength λ₁ and theinterference beam B_(mr1) of the ±1st-ordered diffracted lightcomponents having the wavelength λ₁ reach the photoelectric elementsDTR. When the filter for selecting the wavelength λ₂ is used, theinterference beams ±B_(2r) of the 0th- and −2-order diffracted lightcomponents having the wavelength λ₂ and the interference beam Bmr₂ ofthe ±1st-order diffracted light components having the wavelengths λ₂reach the photoelectric elements DTR. For this reason, in use of thefilter for selecting the wavelength λ₁, the photoelectric signalsI_(mr1), IR02 ₁, and IR20 ₁ are obtained and in use of the filter forselecting the wavelength λ₂, the photoelectric signals I_(mr2), IR02 ₂,IR20 ₂ are obtained.

In the heterodyne scheme, these photoelectric signals appear in the formof a sinusoidal waveform having the same frequency as the beatfrequency. These photoelectric signals are connected to be selectivelyinput to an A/D converter 50 by three switches SS₁ to SS₃ of an analogmultiplexer 120 of the processing circuit shown in FIG. 32. Morespecifically, the switches SS₁ to SS₃ in FIG. 32 are replaced with5-input 1-output switches. Two of the five inputs are used for inputswitching between the set of photoelectric signals I_(mr1), IR02 ₁ andIR20 ₁ and the set of photoelectric signals I_(mr2), IR02 ₂, and IR20 ₂obtained upon detection of the fiducial mark plate FG.

The amplitude values of these photoelectric signals are obtained by theamplitude detection circuit 58 in FIG. 32 and stored. To obtainamplitude ratios, the following arithmetic operations are performed:

C ₁₁ =I _(m1) /I _(mr1)

C ₂₁ =I _(m2) /I _(mr2)

C ₁₂ =IK 02 ₁ /IR 02 ₁

C ₂₂ =IK 02 ₂ /IR 02 ₂

C ₁₃ =IK 20 ₁ /IR 20 ₁

 C ₂₃ =IK 20 ₂ /IR 20 ₂

In this manner, the interference beams of the diffracted lightcomponents passing through the fiducial mark plate are photoelectricallydetected by the photoelectric elements DTR in this embodiment. When thephase information of each photoelectric signal obtained from elementsDTR is compared with that of a photoelectric signal I_(ms) serving asthe reference signal, the position offset of a fiducial mark plate FG,or its position can be measured. That is, part of the baselinemeasurement operation can also serve as the operation for measuring theposition or position offset of the fiducial mark plate FG.

The 14th embodiment of the present invention will be described withreference to FIG. 34. In this embodiment, the polarization directions ofa pair of incident beams +LF and −LF for irradiating a measurement(alignment) grating mark MG on a wafer W (or a fiducial mark plate FG)through an objective lens 22 are set complementary. More specifically,if the incident beams +LF and −LF are linearly polarized beams, theirpolarization directions are set to be perpendicular to each other.However, if the incident beams +LF and −LF are circularly polarizedbeams, they are set to be polarized beams having reverse rotationaldirections. For this reason, the two incident beams ±LF do not interferewith each other, and ±1st-order polarized light components MB ofwavelengths λ₁, λ₂, and λ₃ vertically generated from the grating mark MGdo not interfere with each other.

When the ±1st-order diffracted light components MB are to bephotoelectrically detected through an objective lens 22 and a smallmirror MR2, a polarizing beam splitter PBS serving as an analyzer isused. In this manner, the ±1st-order polarized components BM passingthrough the polarizing beam splitter PBS interfere with each other andserve as a first interference beam B_(p1). The ±1st-order diffractedlight components BM reflected-by the polarizing beam splitter PBSinterfere with each other and serve as a second interference beamB_(p2). The polarization of these interference beams B_(p1) and B_(p2)are complementary. In the heterodyne scheme, the interference beams aresinusoidally intensity-modulated in accordance with the beat frequency.The intensity modulation phases of the interference beams B_(p1) andB_(p2) are different by accurately 180°.

When the linear polarization directions of the incident beams ±LF andthe ±1st-order diffracted light component BM which are perpendicular toeach other are different (rotated) from the polarization separationdirection of the polarizing beam splitter PBS, a λ/2 plate HW shown inFIG. 34 is arranged to correct the linear polarization directions of the±1st-order diffracted light beams BM. For this reason, when the linearpolarization directions of the ±1st-order diffracted light components BMwhich are perpendicular to each other coincide with the polarizationseparation direction of the polarizing beam splitter PBS from thebeginning, or when the incident beams +LF and −LF are circularlypolarized beams having opposite rotational directions, the λ/2 plate HWneed not be used.

In this embodiment, the interference beam B_(p1) is discriminated inunits of wavelengths by dichroic mirrors 32 and 34. The component havingthe wavelength λ₁ of the interference beam B_(p1) is received by aphotoelectric element 36A₁, the component having the wavelength λ₂ isreceived by a photoelectric element 36B₁, and the component having thewavelength λ₃ is received by a photoelectric element 36C₁. Similarly,the wavelengths of the interference beam B_(p2) are discriminated byother dichroic mirrors 32 and 34. The components having the wavelengthsλ₁, λ₂, and λ₃ are respectively received by photoelectric elements 36A₂,36B₂, and 36C₂, respectively.

Output signals from the photoelectric elements 36A. and 36A₂ aresubtracted from each other by a differential amplifier, therebyobtaining a photoelectric signal I_(m1). Output signals from thephotoelectric elements 36B₁ and 36B₂ are subtracted from each other by adifferential amplifier, thereby obtaining a photoelectric signal I_(m2).Output signals from the photoelectric elements 36C₁ and 36C₂ aresubtracted from each other by a differential amplifier, therebyobtaining a photoelectric signal I_(m3).

The differential amplifiers are used as described above because, forexample, the output signal from the photoelectric element 36A₁ and theoutput signal from the photoelectric element 36A₂ have opposite phases(i.e., a phase difference of 180°). The common-phase noise componentsincluded in these outputs are canceled each other to increase thesubstantial S/N ratio of the signal I_(m1). At least the on-axialchromatic aberration of all the aberrations of objective lens 22 (FIG.24, 31, or 34) which are generated in the wavelength range (λ₁ to λ₃) ispreferably corrected to some extent. If the bandwidth of the wavelengthsλ₁ to λ₃ is 100 nm or less, such an on-axial chromatic aberration can becorrected to some extent by selecting proper materials for a pluralityof lens elements constituting the objective lens 22 or combining lenselements having different refractive indices and different dispersionratios. This chromatic aberration need not be perfectly corrected in theobjective lens 22. The chromatic aberration can be corrected byadjustment optical systems 14, 16, and 18 shown in FIG. 24.

Each embodiment has been described above. In detecting the grating markMG on the wafer W or the fiducial mark plate FG in accordance with thehomodyne scheme, the grating mark MG must be prescanned in the pitchdirection to sample the changes in levels of the photoelectric signals.In this case, the simplest scheme is to change a signal waveformsampling clock signal C_(ps) shown in FIG. 25 or 32 into a measurementpulse (e.g., one pulse every 0.02 μm) from an interferometer 44 formeasuring the position of the stage WST. With this arrangement, waveformdata of the respective photoelectric signals generated duringprescanning by several pitches of the grating marks MG are stored in amemory-circuit 54 in correspondence with the positions of the gratingmarks MG.

In the scheme for irradiating the two incident beams ±LF onto thegrating mark MG, the incident beams ±LF are preferably incident on thegrating mark MG at symmetrical angles at least the pitch direction ofthe grating mark MG. In the scheme for projecting one incident beam onthe grating mark MG, as shown in FIG. 23, the incident angle of the beamis preferable zero (vertical incidence) with respect to the pitchdirection of the grating mark MG.

In projecting a multi-wavelength illumination light beam onto themeasurement grating mark MG (or the fiducial mark), the plurality oflaser beams of the respective wavelengths need not be coaxiallysynthesized, as shown in FIGS. 21, 23, and 24, but may be separated inthe non-measurement direction perpendicular to the measurement direction(pitch direction) of the mark position and may be incident separately onthe Fourier transform plane of the grating mark MG. That is, theincident angles of the plurality of illumination beams on the gratingmark MG may be different in the non-measurement direction in units ofwavelengths of the illumination beams.

FIG. 35 shows a state of incidence of beams ±LFλ₁ and ±LFλ₂ on arear-group lens system G2 of the projection lens or the objective lens22. These beams ±LFλ₁ and ±LFλ₂ pass through positions decentered froman optical axis AX on a-Fourier transform plane (pupil plane) EP withrespect to the grating mark MG. The beams ±FLλ₁ or ±LFλ₂ consist of twobeams separated in a direction perpendicular to the drawing surface inFIG. 35. The pitch direction of the grating mark MG in FIG. 35 isperpendicular to the drawing surface. The beams ±LFλ₁ having thewavelength λ₁ are offset from the beams ±LFλ₂ having the wavelength λ₂on the Fourier transform plane EP in the non-measurement direction(i.e., the horizontal direction on the drawing surface of FIG. 35).

With the above arrangement, interference beams B_(m1) and B_(m2) of the±1st-order diffracted light components which are generated from thegrating mark MG and return to the Fourier transform plane EP passthrough positions separated in the non-measurement direction on theFourier transform plane EP in units of wavelengths. The interferencebeam B_(m1) is generated from the mark MG upon irradiation of theincident beams ±LFλ₁. The interference beam B_(m2) is generated from themark MG upon irradiation of the incident beams ±LFλ₂. The incident beamsand the interference beams are distributed on the Fourier transformplane EP, as shown in FIG. 36.

Referring to FIG. 36, when orthogonal axes (measurement andnon-measurement axes) having the center of the Fourier transform planeEP as the origin are set, an offset amount Dh of the two sets ofincident beams ±LFλ₁ and ±LFλ₂ in the non-measurement axis correspondsto the offset amount of the interference beams B_(m1) and B_(m2) of the1st-order diffracted light components in the non-measurement direction.In this manner, when the beams for irradiating the grating mark MG areinclined in the non-measurement direction in units of wavelengthcomponents, the interference beams B_(m1) and B_(m2) are separated anddistributed in the Fourier transform plane EP accordingly. Photoelectricdetection can be performed only when the light-receiving surface of eachphotodetector is located on the Fourier transform plane EP or a planeconjugate to the Fourier transform plane EP.

More specifically, when a plurality of interference beams (interferenceof the ±1st-order diffracted light components and interference of the0th- and −2nd-order diffracted light components) to be photoelectricallydetected are separated on the Fourier transform plane EP in units ofwavelengths, these can be individually photoelectrically detectedwithout using any dichroic mirror used in the previous embodiments.Therefore, as a scheme for separating the interference beam to bedetected into the respective wavelength components, use of a wavelengthselection element such as a dichroic mirror or a bandpass filter is notindispensable.

To obtain a multi-wavelength incident beam, light from a halogen lamp ora high-luminance LED may be used in place of light from a laser lightsource. When light from a halogen lamp is used, a wavelength selectionfilter having a predetermined bandwidth is arranged, and light(broad-band light) having a wavelength width of about 20 nm to 100 nmselected by this filter is guided through an optical filter and used. Inthis case, an incident beam for irradiating the grating mark MG on thewafer has an intensity distribution continuous within the selectedwavelength bandwidth. For this reason, it is desirable that aninterference filter (bandwidth: 3 nm to 10 nm) for extracting only aspecific wavelength component may be fixedly or replaceably arranged infront of each photoelectric element in the light reception system.

As described above, in each of the eighth to 14th embodiments associatedwith the second aspect of the present invention, a position detectionmulti-wavelength illumination light beam or a broad-band light beam isobtained, and diffracted light components generated from a positiondetection grating mark on a substrate are independentlyphotoelectrically detected in units of wavelength components. Markposition information is detected for each resultant photoelectricsignal, and the pieces of mark position information are averaged.High-precision position detection can be performed almost free from theinfluences of asymmetry of the marks and irregularities of the thicknessof the resist layer. The photoelectric signals independent in units ofwavelength components can be obtained in photoelectric detection of thediffracted light components from the mark. For this reason, even if theintensities of the illumination light beams are different in units ofwavelength components, the averaging effect using a multi-wavelengthbeam will not be advantageously impaired, unlike in the conventionalcase.

In each of the eighth to 14th embodiments described above, assume thatdiffracted light components to be photoelectrically detected arecomponents of higher order. In this case, when these multi-wavelengthcomponents (e.g., an interference beam of the 0th- and 2nd-orderdiffracted beams) are simultaneously received by a single photoelectricelement, a canceling phenomenon which has occurred in the conventionalcase can be eliminated. Therefore, higher-precision position detectionand alignment than those of the conventional case can be achieved.

In the second aspect of the present invention, the attenuation ratios(amplitude ratios) of the intensity levels of the photoelectricallydetected diffracted light components of the respective wavelengths areobtained. As for diffracted light components whose attenuation ratiosare small and signal amplitudes are relatively large, position detectionis performed using weighted mean calculation. Therefore,higher-precision position detection than that of simple averaging can beobtained.

In each embodiment described above, multi-wavelength incident beams aresimultaneously irradiated on the grating mark MG. However, high-speedshutters may be arranged at the exit side of the light sources LS₁, LS₂,and LS₃ in FIG. 6 or 24, and one of the incident beams having thewavelengths λ₁, λ₂, and λ₃ may be time-serially switched and emitted. Inthis case, some photoelectric elements in the light reception systemneed not be prepared in advance in units of wavelengths, but may becommonly used. In this manner, when the incident beams are time-seriallyoutput in units of wavelength components, the high-speed shuttermechanism must be arranged in the apparatus. However, the number ofphotoelectric elements and the number of parts of the signal processingcircuit (particularly, the number of A/D converters and the number ofmemory chips) can be greatly reduced. In addition, the wavelengthselection filter 24 in the stage WST shown in FIG. 33 can be omitted.

Time-serial switching of incident beams in units of wavelengthcomponents will be described as the 15th embodiment with reference toFIG. 37. The arrangement in FIG. 37 is based on the arrangement in FIG.6. The same reference numerals as in FIG. 6 denote the same or similarparts in FIG. 37.

Referring to FIG. 37, three laser light sources LS₁, LS₂, and LS₃generate laser beams LB₁, LB₂, and LB₃ having different wavelengths λ₁,λ₂, and λ₃, respectively. For example, the laser light source LS₁ is asemiconductor laser light source of λ₁=0.635 μm, the light source LS₂ isa semiconductor laser light source of λ₂=0.690 μm, and the light sourceLS₃ is a semiconductor laser light source of λ₃=0.760 μm.

Laser power supply units each including a drive circuit for supplying astabilized drive current, a compensation circuit for compensating forthe influence caused by a change in temperature of a laser element, or afeedback control circuit for monitoring a variation in oscillationcenter frequency and controlling to feed back a drive current so as tostabilize the wavelength are arranged for the laser light sources LS₁,LS₂, and LS₃, respectively. These laser power supply units control tostart or stop emitting laser beams from the laser light sources LS₁,LS₂, and LS₃ in response to sequential signals C_(s1), C_(s2), andC_(s3) from a switching control circuit TSC.

In this embodiment, the switching control circuit TSC can be programmedin response to a command signal CQ from a controller 62 in FIG. 38 (tobe described later) such that the laser beams emitted from the laserlight sources LS₁, LS₂, and LS₃ can be sequentially switched for everypredetermined irradiation time. For this reason, the number of lightsources which currently emit laser beams at arbitrary time is limited toone of the three laser light sources LS₁, LS₂, and LS₃. Note that theswitching control circuit TSC can change several beam oscillationtimings of the light sources LS₁, LS₂, and LS₃ in accordance with thecontents of the command signal CQ.

These three laser beams LB₁, LB₂, and LB₃ are aligned so as to passthrough the coaxial optical path through dichroic mirrors DCM₄ and DCM₅.One of the three beams is reflected as a beam LB₀ by a mirror M_(Rb) andvertically incident on a rotary radial grating plate RRG. This rotarygrating plate RRG serves as a frequency shifter for changing thefrequency of each diffracted light component of each order in accordancewith an angular velocity in the same manner as in FIG. 7.

First-order diffracted beams ±D₁₁ generated upon irradiation of the beamLB₁ having the wavelength λ₁, 1st-order diffracted beams ±D₁₂ generatedupon irradiation of the beam LB₂ having the wavelength λ₂, or 1st-orderdiffracted beams ±D₁₃ generated upon irradiation of the beam LB₃ havingthe wavelength λ₃ are generated from the grating RG of the rotarygrating plate RRG. A diffraction angle of the 1st-order diffracted beamfor each wavelength is represented as follows:

sin θ_(n)=λ_(n) /Prg

where n is the number of wavelengths, and Prg is the pitch of thegrating RG.

The 1st-order diffracted beams ±D_(1n) are subjected to a predeterminedfrequency shift Δf regardless of the wavelengths. If a velocity at whichthe grating RG of the grating plate RRG crosses the beam LB₀ is definedas V, Δf=V/Prg is obtained. The +1st-order diffracted beam +D_(1n) has afrequency higher than the frequency of the 0th-order diffracted lightcomponent D₀ by Δf, while the −1st-order diffracted beam −D_(1n) has afrequency lower than the frequency of the 0th-order diffracted lightcomponent D₀ by Δf. Therefore, the rotary radial grating plate RRGserves as a frequency shifter.

Incident beams ±LF consisting of one set of the 1st-order diffractedbeams +D_(1n) (n=1, 2, 3) having the three wavelength components and the0th-order diffracted light component D₀ are converted by a collimatorlens 10 such that their principal rays are parallel to each other, asshown in FIG. 37. These beams reach a beam selection member 12 servingas a spatial filter. The beam selection member 12 shields the 0th-orderdiffracted light component D₀ and passes the incident beams ±LF derivedfrom the 1st-order diffracted light components ±D_(1n). The incidentbeams ±LF then reach a beam splitter (half mirror) 20 through adjustmentoptical systems 14, 16, and 18 constituted by plane-parallel glassmembers whose inclination amounts are variable. The adjustment opticalsystem 14 has a function of deflecting the incident beams ±LF withrespect to the optical axis of the lens 10 while the distance betweenthe incident beams +LF and −LF in the Fourier space is kept unchanged.The adjustment optical systems 16 and 18 have functions of individuallyadjusting the incident beams +LF and −LF with respect to the opticalaxis.

The incident beams ±LF are split into two pairs of beams by the beamsplitter 20. One pair of beams is incident on an objective lens 22,while the other pair of beams is incident on a condenser lens (Fouriertransform lens) 26 through adjustment optical systems 24A and 24Bconstituted by plane-parallel glass members.

The incident beams ±LF incident on the objective lens are collimatedinto parallel beams which are then simultaneously irradiated on thegrating MG on a wafer W at symmetrical angles. Interference fringesformed by the interference of the incident beams ±D₁₁ having thewavelength λ₁, interference fringes formed by the interference of theincident beams ±D₁₂ having the wavelength λ₂, or interference fringesformed by the interference of the incident beams ±D₁₃ having thewavelength λ₃ appear on the grating MG. If the three beams LB₁, LB₂, andLB₃ are simultaneously irradiated on the rotary grating plate RRG, thesethree interference fringes appear at the same pitch and the same phase.

The interference fringes are observed as if they are moving on thegrating MG at a constant speed in one direction because of the frequencydifference 2Δf between the incident beams +LF and −LF. This moving speedis proportional to the velocity V of the grating RG of the rotary radialgrating plate RRG. In association with the frequency difference 2Δf, theON time of each of the light sources LS₁, LS₂, and LS₃ controlled by theswitching control circuit TSC is much longer than the period of thefrequency difference 2Δf (beat frequency) and, for example, set to be100 times or more. For example, if the frequency difference 2Δf is 10kHz (period: 0.1 ms), the ON time of each of the three light sourcesLS₁, LS₂, and LS₃ is preferably 10 ms or more.

As can be apparent from FIG. 37, the surface (grating MG) of the wafer Wand the radial grating plate RRG are located conjugate to each other(imaging relationship) by a composite optical system of the collimatorlens 10 and the objective lens 22. For this reason, the images obtainedby the ±1st-order diffracted light components on the grating RG of theradial grating plate RRG are formed on the grating MG of the wafer W.The images (interference intensity distribution) ½ the pitch of thegrating RG are formed because the 0th-order diffracted light componentD₀ is shielded. A pitch Pif of the interference fringes on the wafer Wis ½ the pitch Pmg of the grating MG as in the previous embodiment.

When the above relationship is satisfied, the 1st-order diffracted lightcomponents vertically emerge from the grating MG upon irradiation of theincident beams ±LF. That is, an interference beam BM (one ofinterference beams B_(m1), B_(m2), and B_(m3) having the wavelengths λ₁,λ₂, and λ₃) of the 1st-order diffracted light component verticallygenerated upon irradiation of the incident beam +LF and the 1st-orderdiffracted light component vertically generated upon irradiation of theincident beam −LF is generated. This interference beam BM is a beatlight component intensity-modulated with the frequency 2Δf.

From the another viewpoint, to generate the ±1st-order diffracted lightcomponents (interference beam BM) in the same direction, a distanceDL_(n) between the incident beams ±LF of the respective wavelengths fromthe optical axis on the Fourier transform plane can be set as follows:

DL _(n) =F ₀·sin θ_(n) =±F ₀·λ_(n) /Pmg (n=1, 2, 3)

where F₀ is the focal length of the objective lens 22. The distanceDL_(n) for each wavelength can be adjusted by appropriately setting thepitch of the grating RG of the rotary radial grating plate RRG and thefocal length of the collimator lens 10.

The interference fringes formed on the wafer W are imaged as adiffraction image of the grating RG of the radial grating plate RRG. Inprinciple, if the pitch of the interference fringes obtained by one ofthe wavelength components having the three wavelengths λ₁, λ₂, and λ₃ isan integer multiple of the pitch of the grating mark MG, the pitches ofthe interference fringes for the remaining wavelength components havethe same relationship. The interference fringes obtained in units ofwavelength components perfectly match and are free from phase offsetsand position offsets.

In practice, however, the interference fringes obtained in units ofwavelength components are subjected to position offsets, phase offsets,and pitch offsets in accordance with the degree of chromatic aberrationof the optical systems such as the objective lens 22 and the collimatorlens 10. To correct these offsets, the adjustment optical systems 14,16, and 18 as in FIG. 6 or 24 are used.

The interference beam BM vertically generated from the grating MG uponirradiation of the above interference fringes passes through theobjective lens 22 and the beam splitter 20 and reaches a spatial filter28. This spatial filter 28 is located on or near the Fourier transformplane associated with the objective lens 22. In this embodiment, thespatial filter 28 has an aperture for transmitting only the interferencebeam BM (±1st-order diffracted components). The interference beam BMhaving passed through the spatial filter 28 is converted into a parallelbeam, reflected by a mirror 32, and received by a photoelectric element36A.

The photoelectric element 36A has the same function as a photoelectricelement 36A in FIG. 6 except that the interference beams B_(m1), B_(m2),B_(m3) to be received are intensity-modulated with the beat frequency2Δf. For this reason, the photoelectric signal I_(mn) output from thephotoelectric element 36A has a wavelength whose level sinusoidallychanges at the same frequency as the beat frequency 2Δf during thepresence of the interference beam BM from the grating mark MG, i.e.,during irradiation of the beam from one of the three light sources LS₁,LS₂, and LS₃.

On the other hand, the incident beams ±LF (one set of the 1st-orderdiffracted beams ±D₁₁, ±D₁₂, and ±D₁₃) incident on the condenser lens 26through the beam splitter 20 and the adjustment optical systems 24A and24B constituted by the plane-parallel glass members are superposed andirradiated on a transmission reference grating SG. In this case, thereference grating SG is located conjugate to the rotary radial gratingplate RRG with respect to the composite system of the collimator lens 10and the condenser lens 26. For this reason, one-dimensional interferencefringes are formed on the reference grating SG by the two-beaminterference of the incident beams ±LF. These interference fringes moveat a speed corresponding to the beat frequency 2Δf.

Note that the adjustment optical systems 24A and 24B compensate theinterference fringes generated on the reference grating SG in units ofwavelength components so as not to cause position offsets and pitchoffsets caused by the chromatic aberration of the condenser lens 26.

When the pitch of the reference grating SG and the pitch of theinterference fringes are appropriately determined, the ±1st-orderdiffracted light components generated from the reference grating SGpropagate as an interference beam B_(ms) in the same direction, passthrough a spatial filter 38, and are received by a photoelectric element40. A photoelectric signal I_(ms) from the photoelectric element 40 hasthe wavelength whose level sinusoidally changes at the same frequency asthe beat frequency 2Δf. The signal I_(ms) serves as the reference signalof the heterodyne scheme.

In the above arrangement, the reference grating SG is formed such that achromium layer is deposited on a glass plate and etched to alternatelyform transparent and light-shielding lines. For this reason, an almostideal grating, i.e., a grating having symmetrical amplitudetransmittances, almost free from at least asymmetry as in the gratingmark MG on the wafer W and the problem posed by the resist layer isformed.

The pair of incident beams irradiated on the reference grating SG may beincident beams having one of the wavelengths λ₁, λ₂, and λ₃ so as toobtain a sufficiently high precision. In this manner, when theinterference fringes having the respective wavelengths are sequentiallyformed on the reference grating SG, and the interference beam B_(ms)generated from the reference grating SG is photoelectrically detected ina time-divisional manner in units of wavelengths. In this case, thereference signal corresponding to the wavelength λ₁, the referencesignal corresponding to the wavelength λ₂, and the reference signalcorresponding to the wavelength λ₃ are individually obtained, so thatthe position of the grating mark MG can be measured in units ofwavelengths. Even if the three interference fringes formed on the waferW in units of wavelength components have a predetermined mutual positionoffset (phase offset), this offset can be measured as an offset amountin advance. This will be described in detail later.

The wafer W shown in FIG. 37 is placed on a wafer stage WSTtwo-dimensionally moved within a plane (X-Y plane) perpendicular to theoptical axis of the objective lens 22. The two-dimensional movement onthe stage WST is performed by a drive source 42 including a drive motor.This driving may be based on a scheme for rotating a feed screw by amotor or a scheme for directly moving the stage itself by anelectromagnetic force of a linear motor. In addition, the coordinateposition of the stage WST is sequentially measured by a laserinterferometer 44. The measurement values of the laser interferometer 44are used for feedback control for the drive source 42.

A fiducial (reference) mark plate FG is formed on part of the waferstage WST. A reflection intensity grating (the pitch is equal to that ofthe grating MG on the wafer) having a line-and-space pattern with achromium layer on the surface of quartz glass is formed on the markplate FG.

For this reason, unlike the phase grating such as the grating mark MGformed by corrugations on the wafer W, the intensity grating ischaracterized in that no asymmetry is present and the diffractionefficiency does not depend on the wavelength of the illumination lightbeam (detection light beam), i.e., in that the amplitude reflectancedoes not have asymmetry. In addition, the reflectance of the chromiumlayer rarely changes in the wavelength band (generally 0.5 μm to 0.8 μm)of the position detection illumination light beam. Therefore, when theintensity grating on the reference mark plate FG is used, the changes inamplitudes of the photoelectric signals I_(m1), I_(m2), and I_(m3) fromthe photoelectric element 36A which are obtained in units of wavelengthsand their amplitude ratios can be accurately obtained.

In the arrangement shown in FIG. 37, semiconductor lasers are used asthe light sources. In this case, it is preferable that an astigmatismremoval shaping optical system (i.e., a plurality of inclinedplane-parallel glass members) be arranged between each of thesemiconductor lasers LS₁, LS₂, and LS₃ and each of the dichroic mirrorsDCM₄ and DCM₅, and the diameters of the beam components of onesynthesized beam LB₀ be equal to each other in units of wavelengthcomponents. In addition, it is preferable that a beam shaping opticalsystem for aligning the diameters of the wavelength components of thesynthesized beam LB₀ be arranged.

For the sake of descriptive simplicity, the rotary radial grating plateRRG is used as the frequency shifter in FIG. 37. However, twoacousto-optic modulators (AOMs) as shown in reference (E) (JapanesePatent Application Laid-open No. 6-82215) may be used, or a first Zeemanlaser light source for oscillating a laser beam having a centerfrequency λ₁ and a second Zeeman laser source for oscillating a laserbeam having a center frequency λ₂ may be used as light sources. In useof a Zeeman laser, it generally oscillates two laser beams whosepolarization directions are complementary, and a frequency difference ofseveral hundreds kHz to several MHz is given between these two laserbeams. The beat frequency of an interference beam to bephotoelectrically detected is increased to a degree corresponding to thefrequency difference. The photoelectric elements 36A and 40 must beconstituted by PIN photodiodes or photomultipliers having a highresponsibility.

Various dichroic mirrors shown in FIG. 37 may be replaced withdispersion elements such as prisms. In this case, one prism has the samefunction as the set of two dichroic mirrors DCM₄ and DCM₅.

An arrangement of a position detection/control circuit suitable for theapparatus shown in FIG. 37 will be described with reference to FIG. 38.The arrangement in FIG. 38 is based on the arrangement of the controlcircuit in FIG. 25. The same reference numerals as in FIG. 25 denotecircuit blocks having the same functions in FIG. 38.

In the heterodyne scheme shown in FIG. 37, while the interference beamBM is generated from the grating mark MG on the wafer W or the referencemark plate FG, the signals I_(m1), I_(m2), and I_(m3) from thephotoelectric element 36A and the signal I_(ms) from the photoelectricelement 40 have sinusoidal AC waveforms shown in FIGS. 39A to 39D.

Note that the signals I_(m1), I_(m2), and I_(m3) shown in FIGS. 39A to39C are signals obtained when the laser light sources LS₁, LS₂, and LS₃are kept on in response to the signals C_(s1), C_(s2), and C_(s3) fromthe switching control circuit TSC in FIG. 37. The reference signalsI_(ms) are output separately (time-divisionally) in units of wavelengthsin response to the ON operations of the laser light sources LS₁, LS₂,and LS₃. These reference signals are represented by one signal waveformin FIG. 39D.

FIG. 39D shows time changes in intensity of the signal I_(ms) serving asthe reference signal. FIGS. 39A, 39B, and 39C show time changes inintensities of the signals I_(m1), I_(m2), and I_(m3) upontime-divisional reception of the interference beam MG from the gratingmark MG on the wafer W in units of wavelengths. Assume that the phase ofthe signal I_(ms) serves as a reference phase. In this case, the phaseof the signal I_(m1), is offset −Δψ₁ with respect to the signal I_(ms),the phase of the signal I_(m2) is offset −Δψ₂ with respect to the signalI_(ms), and the phase of the signal I_(m3) is offset +Δψ₃ with respectto the signal I_(ms). The amplitudes (AC component peak-to-peak values)of the signals I_(m1), I_(m2), and I_(m3) are defined as E₁, E₂, and E₃,respectively.

In the circuit blocks shown in FIG. 38, the photoelectric signal I_(mn)from the photoelectric element 36A and the photoelectric signal I_(ms)from the photoelectric element 40 are input to an analog-to-digitalconversion (A/D converter) circuit unit 50. The instantaneous intensitylevel of each signal is converted into a digital signal in response tothe clock signal (pulse) C_(ps) from a sampling clock generator 52.

The clock generator 52 controls the output timing of the clock signalC_(ps) in response to the command signal CQ transmitted from a positioncontroller 62 (to be described later) to the switching control circuitTSC in FIG. 37. This timing is determined such that the clock signalC_(ps) is always output during the ON state of any of the three lightsources LS_(n) (n=1, 2, 3) in response to a signal C_(sn) (n=1, 2, 3)from the switching control circuit TSC.

The frequency of the clock signal C_(ps) is determined to be much higherthan the beat frequency (2Δf) of the signals I_(mn) (n=1, 2, 3) andI_(ms). The clock signal C_(ps) is also sent to a waveform memorycircuit unit 54 and is used to update the memory address in storing thedigital values (data) from the A/D converter 50.

At this time, the memory circuit unit 54 switches the storage area(address area) of each digital waveform data of the photoelectricsignals I_(mn) and I_(ms) in response to the command signal CQ incorrespondence with the ON light source. For example, six 8-kbyte RAMareas M1A, M1B, M2A, M2B, M3A, and M3B are assured as the waveform datastorage space in the memory circuit unit 54. The digital waveform dataof the signal I_(mn) (I_(m1)) output from the A/D converter 50 duringthe ON state of the light source LS₁ having the wavelength λ₁ issequentially stored in the RAM area M1A in response to the clock signalC_(ps). At the same time, the digital waveform data of the signal I_(ms)output from the A/D converter 50 is sequentially stored in the RAM areaMlB.

While the light source LS₂ is kept on in response to the command signalCQ, the digital waveform data of the signal I_(mn) (I_(m2)) issequentially stored in the RAM area M2A. At the same time, the digitalwaveform data of the signal I_(ms) is sequentially stored in the RAMarea M2B. When the light source LS₃ is kept on, the digital waveformdata of the signal I_(mn) (I_(m3)) is sequentially stored in the RAMarea M3A. At the same time, the digital waveform data of the signalI_(ms) is sequentially stored in the RAM area M3B. In this manner, theRAM areas are switched.

In the three RAM areas MnA (n=1, 2, 3) in the memory circuit unit 54,the waveform data of the signals I_(mn) (n=1, 2, 3) shown in FIGS. 39A,39B, and 39C are digitally sampled for predetermined periods (e.g., 10periods or more). In the three RAM areas MnB (n=1, 2, 3) in the memorycircuit unit 54, the waveform data of the signals I_(ms) shown in FIG.39D are digitally sampled for the same predetermined periods as those ofthe signals I_(mn).

At this time, the waveform data of the three measurement signals Imn inthe waveform memory circuit unit 54 have different timings along thetime axis. Each of the three measurement signals I_(mn) and thecorresponding reference signal I_(ms) are simultaneously sampled. Whenphase differences Δψ₁, Δψ₂, and Δψ₃ of the waveform data of the threemeasurement signals I_(mn) with reference to the waveform data of thereference signals I_(ms), the position offset amounts of the gratingmark MG at the wavelengths λ₁, λ₂, and λ₃ can be accurately obtained.

Note that when the rotary radial grating plate RRG is used, the clocksignal C_(ps) has a frequency of about ten several kHz because theseveral kHz are the upper limit of the beat frequency. As in reference(E) (Japanese Patent Application Laid-open No. 6-82215), when thefrequency shifter constituted by the two AOMs arranged in tandem witheach other is used, the beat frequency is determined by twice thedifference between the frequencies of the high-frequency modulationsignals applied to the respective AOMs and can be relatively freelydetermined.

The waveform data in the memory circuit unit 54 shown in FIG. 38 areinput to a phase difference Δψ_(n), (n=1, 2, 3) and phase offset ΔX_(n)(n=1, 2, 3) detection circuit unit 56. The phase differences Δψ₁, Δψ₂,and Δψ₃ shown in FIGS. 39A, 39B, and 39C are calculated by digitalarithmetic operations (Fourier integration). As previously assumed, ifthe pitch Pmg of the grating mark MG of the wafer W and the pitch Pif ofthe interference fringes irradiated thereon satisfy condition Pmg=2Pif,one period of each of the waveforms shown in FIGS. 39A to 39Ccorresponds to Pmg/2.

Phase difference measurement is performed within the range of ±180°. Thedetection circuit 56 converts the calculated phase differences Δψ₁, Δψ₂,and Δψ₃ into position offset amounts ΔX₁, ΔX₂, and ΔX₃ within the rangeof ±Pmg/4 in accordance with equation (7). These offset amounts ΔX_(n)represent offsets of the grating mark MG within the range of ±Pmg/4 withrespect to the reference grating SG.

Assuming that the resolution of the phase difference measurement isgiven as about 0.2°, the resolution of the offset amount is about(0.2/180)Pmg/4. If the pitch Pmg is set to be 4 μm, a practical range of0.002 μm (2 nm) is obtained.

On the other hand, a signal amplitude and amplitude ratio detectioncircuit unit 58 reads out the waveform data (FIGS. 39A to 39C) stored inthe three RAM areas MnA (n=1, 2, 3) in the waveform memory circuit unit54 and detects amplitude values E₁, E₂, and E₃ of the respective signalsImn by digital arithmetic operations.

The detection unit 58 prestores amplitude values A₁, A₂, and A₃ of thephotoelectric signals I_(m1), I_(m2), and I_(m3) obtained when theinterference beam BM generated from the fiducial mark plate FG isreceived by the photoelectric element 36A in advance.

The grating mark of the fiducial mark plate FG is moved below theobjective lens 22 prior to measurement of the grating mark on the waferW, and signals shown in FIGS. 39A, 39B, and 39C are generated by thephotoelectric element 36A and stored in the waveform memory circuit unit54. The amplitude detection circuit 58 then detects the amplitude valuesA₁, A₂, and A₃ and stores them.

At this time, the stop position of the stage WST which corresponds todetection of the mark plate FG is read by the laser interferometer 44,and position offset amounts ΔX_(b1), ΔX_(b2), and ΔX_(b3) of therespective wavelengths are obtained by the offset amount detectioncircuit unit 56. These values can be utilized as data for determining abaseline.

The baseline here means a small mutual error component when the positionoffset amounts ΔX_(b1), ΔX_(b2), and ΔX_(b3) of the grating mark on themark plate FG, which are measured in units of wavelengths, are slightlydifferent from each other. When the interference fringes of therespective wavelengths which are formed by the beams of the wavelengthsλ₁, λ₂, and λ₃ on the fiducial mark plate FG strictly match each otherin the incident system shown in FIG. 37, the values of the positionoffset amounts ΔX_(b1), ΔX_(b2), and ΔX_(b3) on the mark plate FG areperfectly equal to each other.

As a practical problem, however, when the resolution is about 2 nm, itis difficult to adjust the incident system and the detection system soas to match the position offset amounts ΔX_(b1), ΔX_(b2), and ΔX_(b3) inaccordance with the degree of resolution. For this reason, the mutualdifferences between the position offset amounts ΔX_(b1), ΔX_(b2), andΔX_(b3) measured by the mark plate FG are left as offsets (baselineerrors) unique to the alignment system shown in FIG. 37.

The baseline errors are determined by detecting the grating mark MG onthe wafer W and causing the position offset amounts ΔX₁, ΔX₂, and ΔX₃ ofthe respective wavelengths obtained by the detection circuit 56 by theposition offset amounts ΔX_(b1), ΔX_(b2), and ΔX_(b3) previouslydetermined. As an example, since the interference beam B_(ms) obtainedfrom the reference grating SG in the apparatus shown in FIG. 37 isswitched to one of the wavelengths λ₁, λ₂, and λ₃. For this reason, thevalues of ΔX_(b2)−ΔX_(b1)=ΔX_(b21) and ΔX_(b3)−ΔX_(b1)=ΔX_(b31) arecalculated and stored with reference to, e.g., the position offsetamount ΔX_(b1) of the fiducial mark plate FG, which is measured at anyone wavelength, e.g., the wavelength λ₁.

The value of the amount ΔX₂ is corrected and calculated so as to obtainΔλ₂−ΔX₁=ΔX_(b21) with respect to the position offset amounts ΔX₁, ΔX₂,and ΔX₃ measured for the grating mark MG on the wafer W. Subsequently,the amount ΔX₃ is corrected and calculated so as to obtainΔX₃−ΔX₁=ΔX_(b31).

Alternatively, as a simpler method, the position offset amounts ΔX_(b1),ΔX_(b2), and ΔX_(b3) of the fiducial mark plate FG which are obtainedupon switching between the wavelengths of the interference beam B_(ms)may be stored, and the measured position offset amounts ΔX₁, ΔX₂, andΔX₃ of the grating mark MG on the wafer may be corrected as ΔX₁−ΔX_(b1),ΔX₂−ΔX_(b2), and ΔX₃−ΔX_(b3).

The amplitude ratio detection circuit unit 58 calculate ratios C₁, C₂,and C₃ of the amplitude values E₁, E₂, and E₃ obtained upon detection ofthe grating mark MG on the wafer W to the prestored amplitude values A₁,A₂, and A₃ as C₁=E₁/A₁, C₂=E₂/A₂, and C₃=E₃/A₃. The ratios C₁, C₂, andC₃ correspond to the weighting factors described with reference to theembodiment shown in FIG. 25.

The calculated position offset amounts ΔX₁, ΔX₂, and ΔX₃ and the ratiosC₁, C₂, and C₃ are supplied to a weighted mean calculation circuit unit60, thereby calculating the weighted offset amount ΔX of the gratingmark MG as follows:

ΔX=(C ₁ ·ΔX ₁ +C ₂ ·ΔX ₂ +C ₃ ·ΔX ₃)/(C ₁ +C ₂ +C ₃)

The resultant offset amount ΔX is an offset of the grating mark MG withrespect to the reference grating SG in the pitch direction. This data issupplied to the position controller (display controller) 62 and to aservo control circuit unit 64 when the wafer W is aligned (positioned)in real time.

This servo control circuit unit 64 has two functions. One function is toperform feedback control for the drive source 42 until the offset amountΔX reaches a predetermined value (direct servo mode). To perform thisfunction, the A/D converter 50, the memory circuit unit 54, the offsetamount detection circuit unit 56, and the circuit unit 60 are repeatedlyoperated to calculate the offset amount ΔX every very short period oftime (e.g, several msec.). Note that the calculations of the ratios C₁,C₂, and C₃ may be calculated once by the amplitude ratio detectioncircuit unit 58, or may be calculated every time the offset amount ΔX iscalculated. When the ratios C₁, C₂, and C₃ are calculated every time,the values of the ratios C₁, C₂, and C₃ slightly change every time thecircuit unit 60 calculates the offset amount ΔX, as a matter of course.When the ratios C₁, C₂, and C₃ are to be calculated once or a pluralityof the number of times, the calculated ratio values are used duringsubsequent detection of the same grating mark MG.

The other function of the servo control circuit unit 64 is a function ofmoving the wafer stage WST on the basis of a measurement value from thelaser interferometer 44 (interferometer servo mode). This function isused to position the grating of the fiducial mark plate FG on the stageWST or the grating mark MG on the wafer W immediately below theobjective lens 22, or positioning an arbitrary point on the wafer Wimmediately below the objective lens 22 with reference to the detectedposition of the grating mark MG.

In addition to the servo mode switching instruction, the positioncontroller 62 also has a function of displaying the coordinate positionof the grating mark MG and the calculated offset amount ΔX. In somecase, the position controller (display controller) 62 stores and holdsthe ratios C₁, C₂, and C₃ serving as the weighting factors upondetection of the grating mark MG. Assume that a large number ofidentical grating marks MG are formed on the wafer W, and that thepositions of these marks MG are to be sequentially detected. In thiscase, when the ratios C₁, C₂, and C₃ are sequentially stored, a specificmark MG on the wafer W which causes asymmetry and nonuniformity of theresist layer can be checked.

Portions on the wafer W where the weighting factor (ratios C₁, C₂, andC₃) greatly change may be graphically displayed. At this time, when thechanges in weighting factors are obtained by mounting a wafer prior tocoating of the resist layer after the chemical process such as diffusionand etching in the apparatus of FIG. 37, the influences of the chemicalprocess on the wafer surface can also be indirectly checked. Inaddition, when the resist layer is formed on the wafer to measurechanges in weighting factors, and these changes are compared with thechanges in weighting factors prior to coating of the resist layer, theinfluence of the resist layer can also be indirectly checked.

In the 15th embodiment, the fiducial mark plate FG is placed on thestage WST and used to obtain the rates of changes in signal amplitudesof the respective wavelengths, i.e., the ratios C₁, C₂, and C₃.Therefore, as shown in FIG. 21, photoelectric elements for directlydetecting the intensities of the beams Bri and B_(r2) of the incidentbeams LB₁ and LB₂ need not be arranged.

As previously described, when the fiducial mark plate FG having thechromium surface having a known reflectance is fixed on the wafer stageWST, this is used for measurements of various baseline amounts of themark plate FG and the focus states. Therefore, the position detectionapparatus according to the 15th embodiment shown in FIG. 37 can equallybe applied to, a projection exposure apparatus which requires baselineamount measurement as shown in FIGS. 15 and 16.

When the position detection apparatus of the heterodyne scheme shown inFIG. 37 is to be incorporated in an alignment system TTLA shown in FIG.16, when the incident beams ±LF are switched between a plurality ofwavelength components (these components are separated from each other byabout 30 nm to 40 nm), the crossing region of the beams ±LF irradiatedon the wafer is shifted in the Z direction or the X and Y directions dueto the influence of the chromatic aberration (on-axial factor andmagnification factor) or the influence of the chromatic aberration of anobjective lens OBJ.

A correction optical system CG for correcting the errors generated inaccordance with the chromatic aberrations is arranged in the opticalpath of the incident beams ±LF, as shown in FIGS. 17A and 17B.

FIG. 40 shows the arrangement of a position detection apparatusaccording to the 16th embodiment of the present invention totime-divisionally switch the illumination beams. The basic arrangementof the 16th embodiment is similar to that in FIG. 21. The same referencenumerals as in FIG. 21 denote the members having the same functions inFIG. 40. In this case, a relative position offset amount between twodiffraction grating marks RG and MG in the pitch direction (X direction)is measured by the homodyne scheme. Beams LB₁ and LB₂ as theillumination light beams having different wavelengths λ₁ and λ₂synthesized coaxially as a parallel beam, and vertically irradiated onthe grating mark RG through a beam splitter BS and a mirror MR1.

The beam splitter BS divides the amplitudes of some beams (about several%) B_(r1) and B_(r2) of the beams LB₁ and LB₂ and guides them to aphotoelectric element DT₁. When the photoelectric element DT₁ receivethe beam B_(r1) of the wavelength λ₁ upon irradiation of the mainillumination beams LB₁ and LB₂, the photoelectric element DT₁ outputs aphotoelectric signal I_(r1) representing its intensity value. When thephotoelectric element DT₁ receives the beam B_(r2) of the wavelength λ₂upon irradiation of the main illumination beam LB₂, the photoelectricdetector DT₁ outputs a photoelectric signal I_(r2) representing itsintensity value. A plurality of diffracted light components aregenerated from the grating RG as in FIG. 21 upon irradiation of thebeams LB₁ and LB₂ (parallel light beams).

FIG. 40 illustrates 1st-order diffracted beams +D₁₁ and −D₁₁ generatedfrom the beam LB₁ having the wavelength λ₁, 1st-order diffracted beams+D₁₂ and −D₁₂ generated from the beam LB₂ having the wavelength λ₂, anda 0th-order diffracted beam D₀. Diffracted light components of higherorder can be generated for the beams LB₁ and LB₂ of the respectivewavelengths. For the sake of descriptive simplicity, only the 1st-orderdiffracted beams are illustrated.

The diffracted beams are incident on an imaging optical system dividedinto a front-group lens system G1 and a rear-group lens system G2.

The 0th-order diffracted beam D₀ from the grating RG is shielded by asmall mirror MR2 located at the center of a Fourier transform plane(pupil plane) EP and is prevented from being incident on the rear-grouplens system G2. The 1st-order diffracted beams are parallel beams as inthe beams LB₁ and LB₂ when they emerge from the grating RG, but areconverged as a beam waist at the position of the Fourier transform planeEP by the behavior of the front-group lens system G1.

If the pitch of the grating RG is defined as Prg, the first-orderdiffracted beams ±D₁₁ generated from the beam LB₁ having the wavelengthλ₁ and the first-order diffracted beams ±D₁₂ generated from the beam−LB₂ having the wavelength λ₂ are superposed as parallel beams, throughthe rear-group lens system G2, on a measurement reflection grating MGformed by corrugations on the object side. In this case, the pitchdirection of the grating MG coincides with the X direction.One-dimensional interference fringes (the pitch direction is the Xdirection) of the wavelength λ₁ by two-beam interference of the1st-order diffracted beams ±D₁₁ or one-dimensional interference fringes(the pitch direction is the X direction) of the wavelength λ₂ bytwo-beam interference of the 1st-order diffracted beams ±D₁₂ are formedon the grating MG.

Since the wavelengths λ₁ and λ₂ are different from each other, the±1st-order diffracted beams ±D₁₁ do not interfere with the ±1st-orderdiffracted beams ±D₁₂ even upon simultaneous irradiation of the two mainillumination beams LB₁ and LB₂. It is important that the interferencefringes having the wavelengths λ₁ and λ₂ generated by the 1st-orderdiffracted beams ±D₁₁ and ±D₁₂ appear as one set of interference fringeshaving the same intensity distribution pitch.

An intensity distribution pitch Pif of the interference fringes isdetermined by a pitch Prg of the grating mark RG and a magnificationfactor M of the imaging optical system (G1 and G2) and is defined asPif=M·Prg/2. For example, when the pitch Prg is set to be 4 μm, and themagnification factor M is ¼ (the pattern size of the grating RG isreduced into ¼ on the grating mark MG side), the pitch Pif of theinterference fringes becomes 0.5 μm. If the pitch Pmg of the grating MGto be measured is given as Pmg=2Pif, i.e., Pmg=M·Prg, then rediffractedlight components having the 1st-order diffracted beams ±D₁₁ as theincident light beams are generated.

For example, one rediffracted light component generated by the gratingmark MG upon irradiation of the 1st-order diffracted beam +D₁₁ as theincident beam is the −1st-order diffracted light component (wavelengthλ₁) vertically propagating from the grating mark MG. One rediffractedlight component generated from the grating mark MG upon irradiation ofthe 1st-order diffracted beam −D₁₁ as the incident beam is the+1st-order diffracted light component (wavelength λ₁) verticallypropagating from the grating mark MG. The ±1st-order diffracted lightcomponents of the wavelength λ₁ vertically propagating from the gratingmark MG have an interference intensity corresponding to the mutual phasestates, serve as an interference beam BM (B_(m1)), and reach the mirrorMR2.

Similarly, rediffracted light components are generated from the gratingmark MG upon irradiation of the 1st-order diffracted beams ±D₁₂ as theincident beams. The −1st-order diffracted light component (wavelengthλ₂) generated from the grating mark MG upon irradiation of the 1st-orderdiffracted beam +D₁₂ propagates in a direction perpendicular to thegrating mark. The +1st-order diffracted light component (wavelength λ₂)generated from the grating mark MG upon irradiation of the 1st-orderdiffracted beam −D₁₂ propagates in a direction perpendicular to thegrating MG. The ±1st-order diffracted light components of the wavelengthλ₂ propagating in the direction perpendicular to the grating mark MGalso have an interference intensity corresponding to the mutual phasestates, serve as an interference beam MB (B_(m2)), and reach the mirrorMR2. That is, the interference beam BM becomes the interference beamB_(m1) of the wavelength λ₁ or the interference beam B_(m2) of thewavelength λ₂ in response to switching between the main illuminationbeams LB₁ and LB₂. The interference beam BM is reflected by the mirrorMR2 and reaches the photoelectric element DT₀ through a lens system G3constituting a photoelectric detection system.

The photoelectric element DT₀ outputs, to circuit units CU₁ and CU₃, thephotoelectric signal output during irradiation of the main illuminationbeam LB₁ as the photoelectric signal I_(m1) having a level correspondingto the intensity of the interference beam B_(m1). The photoelectricelement DT₀ outputs, to circuit units CU₂ and CU₄, the photoelectricsignal output during irradiation of the main illumination beam LB₂ asthe photoelectric signal I_(m2) having a level corresponding to theintensity of the interference beam B_(m2).

Note that, for the sake of descriptive convenience for explaining thefunctions of the signal processing circuit, the circuit units CU₁ andCU₂ are separately, illustrated and the circuit units CU₃ and CU₄ areseparately illustrated. However, in the practical circuit arrangement,since the main illumination beams LB₁ and LB₂ are time-divisionallyswitched, one of the circuit units CU₁ and CU₂ and one of the circuitunits CU₃ and CU₄ may be required.

The circuit unit CU₁ obtains the ratio C₁ of the amplitude value of thephotoelectric signal I_(m1) to the signal I_(r1) from the photoelectricelement DT₁ in accordance with the arithmetic operation ofI_(m1)/I_(r1). The circuit unit CU₂ obtains the ratio C₂ of theamplitude value of the photoelectric signal I_(m2) to the signal I_(r2)from the photoelectric element DT₂ in accordance with the arithmeticoperation of I_(m2)/I_(r2). These ratios C₁ and C₂ are output to thecircuit unit CU₅ for calculating the weighted mean as in FIG. 21.

Since the homodyne scheme is employed in this embodiment, theintensities of the interference beams B_(m1) and B_(m2) change inaccordance with relative changes in positions of the gratings RG and MGin the X direction. If the gratings RG and MG are kept stopped, thelevels of the signals I_(m1) and I_(m2) keep predetermined values. Theinterference fringes generated by the grating RG on the grating MG andthe grating MG are relatively scanned a predetermined amount (pitch Pifor more of the interference fringes) in the X direction, the peak andbottom values in the sinusoidal level changes of, the signals I_(m1) andI_(m2) during relative scanning are sampled, and difference values areused as amplitude values in the arithmetic operations of the circuitunits CU₁ and CU₂.

The level change of the signal I_(m1) (this also applies to the signalI_(m2)) which corresponds to the change in positional relationshipbetween the interference fringes and the grating MG is the same asdescribed with reference to FIGS. 22A to 22D, and a detailed descriptionthereof will be omitted.

The circuit units CU₃ and CU₄ calculate X-direction position offsetamounts ΔX₁ and ΔX₂ between the interference fringes and the grating MGon the basis of the amplitude values of the signal I_(m1) and I_(m2) andpreset functions or conversion formulas F(I_(m1)) and F(I_(m2)). Theposition offset values ΔX₁ and ΔX₂ are obtained as values falling withinthe range of ±Pmg/4 using the peak or bottom points of the signalsI_(m1) and I_(m2) as references (origins).

In this embodiment, the main illumination beams LB₁ and LB₂ aretime-divisionally switched and irradiated, the X-direction fine movementstart position of the grating mark MG (wafer) at the time of detectionof a change in level of the photoelectric signal I_(m1) upon irradiationof the main illumination beam LB₁ is set to coincide the X-directionfine movement start position of the grating mark MG (wafer) at the timeof detection of a change in level of the photoelectric signal I_(m2)upon irradiation of the main illumination beam LB₂.

In the practical apparatus arrangement, the coordinate position of themovable stage for moving the grating mark MG is sequentially measured bya measuring unit (e.g., an interferometer) having a resolution muchhigher than the grating pitch Pmg, and the movable stage isservo-controlled on the basis of the measurement values of the measuringunit, thereby reproducing the fine movement start position. As a morepreferable arrangement, changes in levels of the photoelectric signalI_(m1) and I_(m2) are A/D-converted in response to the positionmeasurement position pulses (e.g., one pulse for every 0.02-μmmovement), and a waveform storage means is arranged to sequentiallystore the digital values of the converted levels in a waveform memorycircuit (e.g., a RAM) in which the addresses correspond to the movementpositions.

With the above arrangement, only when the stored waveform data is readout, the amplitude values of the signal I_(m1) and I_(m2) and the bottomor peak coordinate values serving as the reference points on the signalwaveforms can be immediately obtained. The functions or conversionformulas F(I_(m1)) and F(I_(m2)) are sine or cosine functions becausethe signals I_(m1) and I_(m2) are sinusoidal. As an example, a radianψ_(p) is obtained when a level e₁ of the signal I_(m1) at a position tobe detected is defined as follows:

(E _(p1) +E _(b1))/2+{(E _(p1) −E _(b1))sin ψ₁}/2=e ₁

where E_(p1) is the peak level of the signal I_(m1), E_(b1) is thebottom level of the signal I_(m1). The substitution of the radian ψ₁into the following conversion formula using the value of the pitch Pmgyields the offset amount ΔX₁ from the reference point.

ΔX ₁ =Pmg·ψ ₁/4π

Similarly, the offset amount ΔX₂ from the reference point using thesignal I_(m2) is calculated. The offset amounts ΔX₁ and ΔX₂ are suppliedto the circuit unit CU₅ for calculating the weighted mean. The followingarithmetic operation is performed using the ratios C₁ and C₂ as theweighting factors:

ΔX=(C ₁ ·X ₁ +C ₂ ·ΔX ₂)/(C ₁ +C ₂)

The offset amount ΔX obtained in the above arithmetic operation is thefinal position offset amount of the grating MG with respect to thegrating RG.

As can be apparent from the above calculation formula, the offset amountΔX is determined such that the measurement result of the position offsetamount obtained using an interference beam having a wavelength of ahigher intensity in the interference beam BM is weighted with a largerweighting factor. As described above, according to this embodiment, thebeams LB₁ and LB₂ having two different wavelength components are used toirradiate the gratings RG and MG, and the respective wavelengthcomponents of the interference beam BM to be received arephotoelectrically detected, and the weighted mean value is obtained inaccordance with the amplitudes of the received wavelength components.Therefore, a higher-reliability position detection result can beobtained.

In the optical arrangement shown in FIG. 40, assume that the grating RGserves as a grating mark on the-mask, that the grating MG serves as amark on the wafer, and that the imaging systems G1 and G2 are projectionlenses for projecting the mask pattern on the wafer. In this case, analignment device in the projection exposure apparatus can be realized.

FIG. 41 shows the schematic arrangement of the 17th embodiment. Thebasic arrangement of the 17th embodiment is the same as that of FIG. 23,and the same reference numerals as in FIG. 23 denote the same parts inFIG. 41. In the 17th embodiment, two illumination beams LB₁ and LB₂ areincident on a mirror MR2 located at the center of the pupil plane of animaging optical system (G1 and G2) through a lens system G4. The beamsLB₁ and LB₂ deflected downward by the mirror MR2 are time-divisionallyswitched and converted into a parallel beam through a rear-group lenssystem G2 and vertically irradiated on a grating MG. First-orderdiffracted beams ±D₁₁ of a wavelength λ₁ diffracted by the grating MG or1st-order diffracted beams ±D₁₂ of a wavelength λ₂ diffracted by thegrating MG cross (imaging) on the grating RG through the lens systems G1and G2. Since the grating RG is of a transmission type, the ±1st-orderdiffracted light components of the rediffracted light beams from thegrating RG upon irradiation of the 1st-order diffracted beams ±D₁₁ or±D₁₂ propagate in a direction opposite to the imaging optical system andperpendicular to the grating RG. The 1st-order diffracted lightcomponents become an interference beam BM (one of an interference beamB_(m1) having the wavelength λ₁ and an interference beam B_(m2) havingthe wavelength λ₂) through a mirror MR3, an alignment objective lens G5,and a spatial filter 28, and the interference beam BM is incident on aphotoelectric element DT₀. The remaining arrangement (i.e., thephotoelectric element DT₁ and the circuit units CU₁, CU₂, CU₃, CU₄, andCU₅) is the same as in FIG. 40.

The photoelectric element DT₀ outputs one of the photoelectric signalsI_(m1) and I_(m2) in accordance with time-divisional switching of themain illumination bemas LB₁ and LB₂.

This embodiment has the relationship between beam incidence and beamreception which is opposite to that in FIG. 40. In the arrangement ofthis embodiment, a grating MG is formed on a semiconductor wafer, agrating RG is formed on a reticle (mask), and this arrangement can beapplied the apparatus of reference (F) (Japanese Patent ApplicationLaid-open No. 3-3224) in which lens systems G1 and G2 are reductionprojection lenses for projection exposure. In the apparatus disclosed inreference (F), the small lens for slightly deflecting 1st-orderdiffracted beams on the pupil plane EP of the projection lens, therebycorrecting the chromatic aberration generated by the projection lens.However, when the embodiment of FIG. 41 is applied, a small lens (e.g.,flint glass having a large color dispersion or an aspherical lens) mustbe arranged to optimally correct 1st-order diffracted beams ±D₁₁ and±D₁₂ having a small wavelength difference.

In the 17th embodiment, since the illumination beams LB₁ and LB₂ aredirectly incident on, e.g., the grating MG on the wafer, the intensitiesof the 1st-order diffracted beams ±D₁₁ and ±D₁₂ generated from thegrating MG can be set higher than the diffracted beams (interferencebeams BM) generated from the grating MG in FIG. 40.

The 18th embodiment of the present invention will be described below. Inthis embodiment, using the arrangement shown in FIG. 37 as a base, aninterference beam of 0th- and 2nd-order diffracted light components froma grating mark are detected in addition to the interference beam of the±1st-order diffracted light components from the grating mark, asdescribed above. In a system wherein the interference beam of the 0th-and 2nd-diffracted light components is photoelectrically converted by asingle photoelectric element to detect a position offset of the gratingmark using this photoelectric signal, when an interference beam(multi-wavelength beam) of the 0th- and 2nd-order diffracted lightcomponents upon obtaining a multi-wavelength incident beam forilluminating a grating mark is received by the single photoelectricelement, it is difficult to properly detect the position offset.

The main reason for this can be easily understood, as described withreference to FIGS. 29A to 29D and 30A to 30D. The phase differencesbetween three photoelectric signals IK02 _(n) (n=1, 2, 3) are largerthan those between the photoelectric signals I_(mn) (FIGS. 39A to 39C)of the interference beams of the ±1st-order diffracted light components.

For this reason, when changes in intensities of the wavelengths havingthe large phase differences are received by the single photoelectricelement, the amplitudes (AC amplitude components) of the photoelectricsignals become very small by the canceling effect of the wavelengthintensities.

The arrangement of this embodiment will be described with reference toFIG. 42. FIG. 42 shows part of the arrangement of FIG. 37 and is amodification of a photoelectric detection system for detecting variousinterference beams from a grating mark MG. The same reference numeralsas in FIG. 37 denote the parts having the same functions in FIG. 42.Referring to FIG. 42, an incident system 100 includes light sources LS₁,LS₂, and LS₃, a mirror MR, dichroic mirror DCM₄ and DCM₅, a radialgrating plate RRG serving as a frequency shifter, a lens 10, a spatialfilter 12, and adjustment optical systems 14, 16, and 18. The incidentsystem 100 emits a pair of incident beams +LF and −LF.

The incident beams ±LF sequentially switched to one of the wavelengthsλ₁, λ₂, and λ₃ are partially reflected by a half mirror 20 and incidenton an objective lens 22. The remaining part of the beams is incident ona reference light reception system 110. The reference light receptionsystem 110 comprises adjustment optical systems 24A and 24B, a lens 26,a reference grating SG, and a spatial filter 38 in FIG. 37. Thereference light reception system 110 guides a reference light beamB_(ms) to a photoelectric element 40. When a grating MG on a wafer W isirradiated with the incident beams ±LF through the objective lens 22, aninterference beam BM of the ±1st-order diffracted light components isvertically generated from the grating MG. At the same time, interferencebeams BM02 and BM20 of the 0th- and 2nd-order diffracted lightcomponents are generated in a direction opposite to the travelingdirection of each incident beam. The interference beam MB of the±1st-order diffracted light components and the interference beams BM02and BM20 of the 0th- and 2nd-diffracted light components are reflectedby a mirror 32 through the objective lens 22 and the half mirror 20. Theinterference beam BM is received by the photoelectric element DT₀, andthe interference beams BM02 and BM20 are received by photoelectricelements DT_(2a) and DT_(2b), respectively.

As previously described, in response to switching to one of thewavelength components of the incident beams ±LF, the interference beamMB becomes one of the interference beam B_(m1) having the wavelength λ₁,the interference beam B_(m2) having the wavelength λ₂, and theinterference beam B_(m3) having the wavelength λ₃. Similarly, theinterference beams BM02 and MB20 are set to one wavelength componenthaving one of the three wavelengths λ₁, λ₂, and λ₃ in response toswitching to one wavelength of the incident beams ±LF.

When the photoelectric elements DT₀, DT_(2a), and DT_(2b) are located onor near the Fourier transform plane of the objective lens 22, theinterference beams BM02 and BM20 of the 0th- and −2nd-order diffractedlight components are laterally shifted on the photoelectric elementsDT_(2a) and DT_(2b) in units of wavelengths. The light-receivingsurfaces of the photoelectric elements DT_(2a) and DT_(2b) have a sizeto cover such the lateral shift. When spatial filters are respectivelyarranged in front of the photoelectric elements DT₀, DT_(2a), andDT_(2b) to select each interference beam, the size of the beam selectionopening must be determined in consideration of this lateral shift.Alternatively, prisms (or plane-paralell glass) made of a glass materialhaving a large color dispersion may be arranged in front of thephotoelectric elements DT_(2a) and DT_(2b) to reduce the lateral shiftscaused by the respective wavelengths on the light-receiving surfaces.

As can be apparent from the above arrangement, a signal processingcircuit for obtaining the phase differences of the photoelectric signalsI_(mn), IK02 _(n), and IK20 _(n) (n=1, 2, 3) from the photoelectricelements DT₀, DT_(2a), and DT_(2b) using the photoelectric signal I_(ms)from the photoelectric element 40 as a reference signal is required inthis embodiment. The simplest circuit arrangement is shown in FIG. 43.

FIG. 43 is an improvement of part of the processing circuit shown inFIG. 38. In the hardware arrangement, an A/D converter 50 in FIG. 38 isconstituted by four channel A/D conversion IC circuits ADCa, ADCb, ADCc,and ADCd. The reference signal I_(ms) and the measurement signalsI_(mn), IK02 _(n), and IK20 _(n) are respective applied to these fourchannels. The four signals can be almost simultaneously digitallysampled in response to a pulse signal C_(ps) from a sampling clockgenerator 52 in FIG. 38.

A waveform memory circuit unit 54 has four channel memory banks MM_(an),MM_(b1), MM_(cn), and MM_(dn) for simultaneously storing the signalwaveform data from the A/D conversion IC circuits ADCa, ADCb, ADCc, andADCd as shown in FIG. 44. The respective banks have memory areas a1 toa3, b1 to b3, c1 to c3, and d1 to d3 corresponding to the number ofwavelengths (in this case, three wavelengths).

The waveform memory circuit unit 54 sequentially switches writablememory areas of the memory banks MM_(an), MM_(bn), MM_(cn), and MM_(dn)so as to synchronize with wavelength switching of the incident beams ±LFin response to a command signal CQ from a position controller 62. Theaddress counters for the 12 memory areas a1 to a3, b1 to b3, c1 to c3,and d1 to d3 are commonly updated in response to the pulse signalC_(ps). However, the number of memory areas simultaneouslywrite-accessed in response to the command signal CQ is four.

More specifically, when the incident beams ±LF is set at the wavelengthλ₁, the digital waveform data of the signal I_(mn), the signal IK02_(n), the signal IK20 _(n), and the reference signal I_(ms) (λ₁) arestored in the memory areas a1, b1, c1, and d1, respectively.

Similarly, when the incident beams ±LF is set at the wavelength λ₂, thedigital waveform data of the signal I_(mn), the signal IK02 _(n), thesignal IK20 _(n), and the reference signal I_(ms) (λ₂) are stored in thememory areas a2, b2, c2, and d2, respectively. When the incident beams±LF is set at the wavelength λ₃, the digital waveform data of the signalI_(mn), the signal IK02 _(n), the signal IK20 _(n), and the referencesignal I_(ms) (λ₃) are back stored in the memory areas a3, b3, c3, andd3, respectively.

An amplitude detection and amplitude ratio detection circuit 58 shown inFIG. 38 is changed to output ratio data C_(n1), C_(n2), and C_(n3) (n=1,2, 3 corresponding to the wavelengths) grouped in units of interferencebeams having different diffracted states in FIG. 43. Of these ratiodata, ratio data C_(n1) (n=1, 2, 3) are identical to ratios C₁, C₂, andC₃ in FIG. 38. The ratio data Cn₂ (n=1, 2, 3) are ratios obtained fromthe photoelectric signals IK02 _(n) (n=1, 2, 3) in units of wavelengths.The ratio data C_(n3) (n=1, 2, 3) are ratios obtained from thephotoelectric signals IK20 _(n) (n=1, 2, 3) in units of wavelengths.

A phase difference and position offset detection circuit 56 shown inFIG. 38 is modified to output offset amounts ΔX_(n1), ΔX_(n2), andΔX_(n3) (n=1, 2, 3) grouped in units of interference beams havingdifferent diffracted states in FIG. 43. Of these offset amounts, theoffset amounts ΔX_(n1) (n=1, 2, 3) are equal to offset amounts ΔX₁, ΔX₂,and ΔX₃ in FIG. 38. The offset amounts ΔX_(n2) (n=1, 2, 3) are obtainedfrom the photoelectric signals IK02 _(n) (n=1, 2, 3) in units ofwavelengths. The offset amounts ΔX_(n3) (n=1, 2, 3) are obtained fromthe photoelectric signals IK20 _(n) (n=1, 2, 3) in units of wavelengths.Note that this detection circuit 56 calculates, as intermediate values,values corresponding to the phase differences Δβ_(0n) and Δβ_(2n) (n=1,2, 3) described in FIGS. 29A to 29D and FIGS. 30A to 30D.

A weighted mean calculation circuit unit 60 in FIG. 38 is modified intoa selective weighted mean calculation circuit in FIG. 43. This circuithas the first arithmetic mode for calculating a final position offsetamount ΔX on the basis of the photoelectric detection results of theinterference beam of the ±1st-order diffracted light components as inFIG. 38, the second arithmetic mode for calculating the final offsetamount ΔX on the basis of the photoelectric detection results of theinterference beams of the 0th- and −2nd-order diffracted lightcomponents, and the third arithmetic mode for calculating the finaloffset amount ΔX on the basis of the detection results of all theinterference beams. These three arithmetic operation modes can bearbitrarily selected by the operator. When the third arithmetic mode isdesignated, a few additional arithmetic algorithms can be selected. Suchmode designation and algorithm designation will be described in detaillater on.

In this embodiment, a wafer stage WST is positioned to irradiateincident beams ±LF from the objective lens 22 onto the grating mark of afiducial mark plate FG on the wafer stage WST.

The three light sources LS₁, LS₂, and LS₃ are sequentially switchedevery predetermined period of time (e.g., about 100 times the period ofthe beat frequency 2Δf) in response to the command signal CQ from theposition controller 62 (FIG. 38). The grating mark of the fiducial markplate FG is irradiated with the incident beams ±LF whose wavelength issequentially switched. For example, if the three light sources LS₁, LS₂,and LS₃ are switched and turned on from the shorter wavelength side, thedigital waveform data of the signal I_(mn) (n=1) output from thephotoelectric element DT₀ during the ON state of the light source LS₁(wavelength λ₁) is stored in the memory area a1 in FIG. 44 in responseto the sampling pulse signal C_(ps).

Simultaneously, the digital waveform data of the signal IK02 _(n) (n=1)from the photoelectric element DT_(2a) is stored in the memory area b1in response to the pulse signal C_(ps). The digital waveform data of thesignal IK20 _(n) (n=1) from the photoelectric element DT_(2b) is storedin the memory area c1 in response to the pulse signal C_(ps). Thedigital waveform data of the signal I_(ms) (wavelength λ₁) from thephotoelectric element 40 is stored in the memory area d1 in response tothe pulse signal C_(ps).

Similarly, the signals I_(ms), IK02 _(n) (n=2, 3), and I_(ms) from thecorresponding photoelectric elements during the ON states of the lightsources LS₂ and LS₃ are stored in the corresponding memory areas a_(n),b_(n), c_(n), and d_(n) (n=2, 3).

The respective waveform data in the memory bank MMb in the memorycircuit unit 54 are analyzed by the amplitude detection circuit 58, andthe amplitude values (peak-to-peak values) of the signal IK02 _(n) ofthe respective wavelengths are calculated as values J02 _(n) (n=1, 2,3). The respectively waveform data in the memory bank MM_(cn) in thememory circuit unit 54 are analyzed by the amplitude detection circuit58, and the amplitude values (peak-to-peak values) of the signal IK20_(n) of the respective wavelengths are calculated as values J20 _(n)(n=1, 2, 3). The respective waveform data in the memory bank MM_(an) areanalyzed to obtain and store amplitude values J11 _(n) (n=1, 2, 3) ofthe signals I_(mn) of the respective wavelengths.

When the preliminary operation is thus completed, the wafer W to beactually positioned and aligned is placed on the stage WST. The stageWST is positioned to irradiate the incident beams ±LF from the objectivelens 22 onto the grating mark MG on the wafer W.

In the same manner as in detection of the grating mark of the fiducialmark plate FG, the three light sources LS₁, LS₂, and LS₃ aresequentially switched and turned on to simultaneously store the waveformdata of the photoelectric signals I_(mn), IK02 _(n), IK20 _(n) (n=1, 2,3), and I_(ms) in the memory circuit unit 54. The amplitude values ofthe signals I_(mn), IK02 _(n), and IK20 _(n) (n=1, 2, 3) stored in thememory areas a_(n), b_(n), and c_(n) of the memory circuit unit 54 arecalculated as E_(n) (see FIGS. 39A to 39D), and E02 _(n) and E20 _(n)(see FIGS. 29A to 29D and 30A to 30D) by the detection circuit 58.

The phase offset detection circuit unit 56 reads out the signal I_(mn),IK02 _(n), and IK20 _(n) (n=1, 2, 3) from the memory areas a_(n), b_(n),and c_(n) of the memory circuit unit 54 in units of wavelengths tosequentially calculate phases Δψ_(n), Δβ_(0n), Δβ_(2n) (n=1, 2, 3) andposition offset amounts ΔX_(n1), ΔX_(n2), and ΔX_(n3) (n=1, 2, 3) of thesignals I_(mn), IK02 _(n), and IK20 _(n) with respect to the referencesignal I_(ms).

When the amplitude values and the position offset amounts of therespective wavelengths are obtained in units of detection light beamshaving different diffracted states (in units of interference beams), theamplitude ratio detection circuit 58 performs the following arithmeticoperations:

 C ₁₁ =E ₁ /J 11 ₁  (A1)

C ₂₁ =E ₂ /J 11 ₂  (A2)

C ₃₁ =E ₃ /J 11 ₃  (A3)

C ₁₂ =E 02 ₁ /J 02 ₁  (B1)

C ₂₂ =E 02 ₂ /J 02 ₂  (B2)

C ₃₂ =E 02 ₃ /J 02 ₃  (B3)

C ₁₃ =E 20 ₁ /J 20 ₁  (C1)

C ₂₃ =E 20 ₂ /J 20 ₂  (C2)

C ₃₃ =E 20 ₃ /J 20 ₃  (C3)

The most probable offset amount ΔX is calculated by the weighted meancalculation circuit 60. In the first arithmetic mode using only theinterference beam BM of the ±1st-order diffracted light components, theoffset amount ΔX is calculated as follows in the same manner as in FIG.38:

ΔX=(C ₁₁ ·ΔX ₁₁ +C ₂₁ ·ΔX ₂₁ +C ₃₁ ·ΔX ₃₁)/(C ₁₁ +C ₂₁ +C ₃₁)

On the other hand, in the second arithmetic mode using only theinterference beam of the 0th- and −2nd-order diffracted lightcomponents, an algorithm is employed to calculate the position offsetamount of each wavelength in accordance with an average phase differencebetween phase differences Δβ_(0n) obtained upon detection of theinterference beam of the 0th- and −2nd-order diffracted lightcomponents, which appears on the left side of the interference beam BMof the ±1st-order diffracted light components and phase differencesΔβ_(2n) obtained upon detection of the interference beam of the 0th- and−2nd-order diffracted light components, which appears on the right sideof the interference beam BM of the ±+1st-order diffracted lightcomponents. This phase difference average is not an average for reducingso-called random components to improve precision, but an average whichmust be obtained in principle in position detection using theinterference beams of the 0th- and ±2nd-order diffracted lightcomponents.

Based on this algorithm of this embodiment, the weighted meancalculation circuit 60 calculates average values ΔXA_(n) (n=1, 2, 3) ofthe respective wavelengths between the position offset amounts ΔX_(n2)(n=1, 2, 3) obtained from the signals IK02 _(n) and the position offsetamounts ΔX_(n3) (n=1, 2, 3) obtained from the signals IK20 _(n) (n=1, 2,3) as follows:

ΔXA ₁=(ΔX ₁₂ +ΔX ₁₃)/2

ΔXA ₂=(ΔX ₂₂ +ΔX ₂₃)/2

ΔXA ₃=(ΔX ₃₂ +ΔX ₃₃)/2

The weighted mean calculation circuit 60 also calculates average valuesCA_(n) (n=1, 2, 3) of the amplitude ratios C_(n2) and C_(n3) of the 0th-and −2nd-order diffracted light components of the respective wavelengthsobtained in the amplitude ratio detection circuit 58 as follows:

CA ₁=(C ₁₂ +C ₁₃)/2

CA ₂=(C ₂₂ +C ₂₃)/2

 CA ₃=(C ₃₂ +C ₃₃)/2

The weighted mean calculation circuit 60 then calculates the weightedmean value of the average position offset amounts ΔXA_(n) using theaverage ratios CA_(n) of the respective wavelength components asweighting factors, thereby calculating the most probable offset amountΔX as follows:

ΔX=(CA ₁ ·ΔXA ₁ +CA ₂ ·ΔXA ₂ +CA ₃ ·ΔXA ₃)/(CA ₁ +CA ₂ +CA ₃)

By the above calculation, position detection and position offsetdetection of the grating mark in the second arithmetic mode can beachieved.

In the third arithmetic mode, the operator can arbitrarily set one ofthe first algorithm for simply averaging the position offset amountcalculated in the first arithmetic mode and the position offset amountcalculated in the second arithmetic mode, and the second algorithm forcalculating the weighted mean of these two position offset amounts. LetΔXM₁ be the position offset amount finally calculated in the firstarithmetic mode (i.e., the mode using the detection results of theinterference beam of the +1st-order diffracted light components) andΔXM₂ be the position offset amount finally calculated in the secondarithmetic mode. In this case, the position offset amount determined bythe first algorithm is calculated as (ΔXM₁+ΔXM₂)/2.

On the other hand, in the second algorithm, the weighted mean value ofthe offset amount ΔXM₁ calculated in the first arithmetic mode and theoffset amount ΔXM₂ calculated in the second arithmetic mode arecalculated with predetermined weighting factors Q₁ and Q₂. As anexample, the weighting factor Q₁ is caused to correspond to the sum ofthe amplitude values E₁, E₂, and E₃ (see FIGS. 39A to 39D) of thesignals I_(mn) (n=1, 2, 3) obtained upon photoelectric detection of theinterference beam BM of the ±1st-order diffracted light components, andthe weighting factor Q₂ is caused to correspond to the sum of theaverage amplitude values (E02 ₁+E20 ₁)/2, (EO2 ₂+E20 ₂)/2, and (EO2₃+E20 ₃)/2 of the signals IK02 _(n) and IK20 _(n) (n=1, 2, 3) obtainedfor photoelectrically detecting the interference beam of the 0th- and−2nd-order diffracted light components in units of wavelengths.Therefore, the offset amount ΔX of the grating mark MG is determined bythe following calculation in the second algorithm:

ΔX=(Q ₁ ·ΔXM ₁ +Q ₂ ·ΔXM ₂)/(Q ₁ +Q ₂)

In principle, diffracted light components of higher order have lowerintensities. For this reason, the light intensity amplitudes(corresponding to E_(n)) of the interference beam BM of the +1st-orderdiffracted light components are much larger than those (corresponding toE02 _(n) and E20 _(n)) of the interference beam of the 0th- and−2nd-order diffracted light components. When the weighting factors Q₁and Q₂ are simply determined by the sums of the amplitudes of thesignals I_(mn), IK02 _(n), and IK20 _(n), the weighting factor Q₁ isusually larger than the weighting factor Q₂. Therefore, the calculatedvalue with respect to the weighting factor Q₂ is preferably corrected toincrease by, e.g., a predetermined ratio (e.g., 10% to 30%).

The 19th embodiment of the present invention will be described withreference to FIG. 45. This embodiment is basically the same as that ofFIG. 33. In the 19th-embodiment, the structure of a fiducial mark plateFG on a wafer stage WST shown in FIG. 37 is replaced with a transmissiongrating (i.e., a grating whose amplitude transmittance does not haveasymmetry). An interference beam transmitted through this grating isphotoelectrically detected to obtain a denominator (reference value)used in causing a detection circuit 58 to calculate the amplitude ratiosof photoelectric signals I_(mn), IK02 _(n), and IK20 _(n). Therefore,the same reference numerals as in FIG. 33 denote the same parts in FIG.45.

FIG. 45 shows the partial section of the wafer stage WST. When incidentbeams ±LF (in this case, the beams have two wavelengths λ₁ and λ₂) areirradiated on the grating of the fiducial mark plate FG, 0th-, ±1st- and±2nd-order diffracted light components are generated from the gratingtoward the interior of the stage.

These diffracted light components are deflected at a right angle by amirror MR and incident on a lens system G5 having a Fourier transformfunction. These beams become interference beams B_(mrn) (n=1, 2) of the±1ar-order diffracted light components and interference beams ±B_(1r)(wavelength λ₁) and ±B_(2r) (wavelength λ₂) which are then incident onphotoelectric elements DTR.

The photoelectric elements DTR comprise a central light-receivingportion for receiving the interference beam B_(mrn) and outputting ACphotoelectric signals I_(mrn) corresponding to the beat frequency, alight-receiving portion for commonly receiving the interference beams+B_(1r) (wavelength λ₁) and −B_(2r) (wavelength λ₂) and outputting ACphotoelectric signal IR20 _(n) (wavelength λ₂) corresponding to the beatfrequency, and a light receiving portion for commonly receiving theinterference beams −B_(1r) (wavelength λ₁) and −B_(2r) (wavelength λ₂)and outputting AC photoelectric signal IR02 _(n) corresponding to thebeat frequency.

When the incident beams ±LF are set at the wavelength λ₁, theinterference beams ±B_(1r) of the 0th- and −2nd-order diffracted lightcomponents having the wavelength λ₁ and the interference beam B_(mr1) ofthe ±1st-ordered diffracted light components having the wavelength λ₁reach the photoelectric elements DTR. When the incident beams ±LF areset at the wavelength λ₂, the interference beams ±B_(2r) of the 0th- and−2-order diffracted light components having the wavelength λ₂ and theinterference beam B_(mr2) of the +1st-order diffracted light componentshaving the wavelengths λ₂ reach the photoelectric elements DTR. For thisreason, in use of the incident beams ±LF having the wavelength λ₁, thephotoelectric signals I_(mr1), IR02 ₁, and IR20 ₁ are obtained. In useof the incident beams ±LF having the wavelength λ₂, the photoelectricsignals I_(mr2), IR02 ₂, and IR20 ₂ are obtained.

In the heterodyne scheme, these photoelectric signals appear in the formof a sinusoidal waveform having the same frequency as the beatfrequency. These photoelectric signals are switched and input to the A/Dconverter 50 shown in FIG. 43 in response to the input signal I_(mn),IK02 _(n), and IK20 _(n).

More specifically, a switch for switching between the signals I_(mrn)and I_(mn) and supplying the selected signal to the A/D converter 50, aswitch for switching between the signals IR02 _(n) and IK02 _(n) andsupplying the selected signal to the A/D converter 50, and a switch forswitching between the signals IR20 _(n) and IK20 _(n) and supplying theselected signal to the A/D converter 50 are added in the circuit shownin FIG. 43. These three switches are synchronously operated in responseto a command signal from the position controller 62 (FIG. 38).

The photoelectric signals from these photoelectric elements DTR aretemporarily stored in the waveform memory circuit unit 54, and theamplitude values of these photoelectric signals are obtained and storedby the amplitude detection circuit unit 58 in FIG. 43. To obtain theiramplitude ratios, the following operations are performed:

C ₁₁ =I _(m1) /I _(mr1)

C ₂₁ =I _(m2) /I _(mr2)

C ₁₂ =IK 02 ₁ /IR 02 ₂

C ₂₂ =IK 02 ₂ /IR 02 ₂

C ₁₃ =IK 20 ₁ /IR 20 ₁

C ₂₃ =IK 20 ₂ /IR 20 ₂

In this manner, the interference beams of the diffracted lightcomponents passing through the fiducial mark plate are photoelectricallydetected by the photoelectric elements DTR in this embodiment. When thephase information of each photoelectric signal obtained from theelements DTR is compared with that of the photoelectric signal I_(ms)serving as the reference signal, the position offset of the fiducialmark plate FG, or its position can be measured. That is, part off thebaseline measurement operation can also serve as the operation formeasuring the position or position offset of the fiducial mark plate FG.

The 20th embodiment of the present invention will be described withreference to FIG. 46. This embodiment is basically the same as that ofFIG. 18. In the 20th embodiment, the polarization directions of a pairof incident beams +LF and −LF for irradiating a measurement (alignment)grating mark MG on a wafer W (or a fiducial mark plate FG) through anobjective lens 22 are set complementary. More specifically, if theincident beams +LF and −LF are linearly polarized beams, theirpolarization directions are set to be perpendicular to each other.However, if the incident beams +LF and −LF are circularly polarizedbeams, they are set to be polarized beams having reverse rotationaldirections. For this reason, the two incident beams ±LF do not interferewith each other, and '1st-order polarized light components MB ofwavelengths λ₁, λ₂, and λ₃ vertically generated from the grating mark MGdo not interfere with each other.

When the ±1st-order diffracted light components MB are to bephotoelectrically detected through an objective lens 22 and a smallmirror MR2, a polarizing beam splitter PBS serving as an analyzer isused. In this manner, the ±1st-order polarized components BM passingthrough the polarizing beam splitter PBS interfere with each other andserve as a first interference beam B_(p1). The ±1st-order diffractedlight components BM reflected by the polarizing beam splitter PBSinterfere with each other and serve as a second interference beamB_(p2).

These interference beams B_(p1) and B_(p2) are complementary. In theheterodyne scheme, the interference beams are sinusoidallyintensity-modulated in accordance with the beat frequency. The intensitymodulation phases of the interference beams B_(p1) and B_(p2) aredifferent by accurately 180°.

When the linear polarization directions of the incident beams ±LF andthe ±1st-order diffracted light component BM which are perpendicular toeach other are different (rotated) from the polarization separationdirection of the polarizing beam splitter PBS, a λ/2 plate HW shown inFIG. 46 is arranged to correct the linear polarization directions of the±1st-order diffracted light beams BM. For this reason, when the linearpolarization directions of the ±1st-order diffracted light components BMwhich are perpendicular to each other coincide with the polarizationseparation direction of the polarizing beam splitter PBS from thebeginning, or when the incident beams +LF and −LF are circularlypolarized beams having opposite rotational directions, the λ/2 plate HWneed not be used.

In this embodiment, the interference beam B_(p1) is received by aphotoelectric element DT_(0a) (36A₁ in FIG. 18), and the interferencebeam B_(p2) is received by a photoelectric element DT_(0b) (36A₂ in FIG.18). The photoelectric signals from the photoelectric elements DT_(0a)and DT_(0b) are subtracted by a differential amplifier, therebyobtaining the photoelectric signal I_(mn). The use of the differentialamplifier results from the fact that the photoelectric signals from thephotoelectric elements DT_(0a) and DT_(0b) have opposite phases (i.e., aphase difference of 180°). The common-phase noise components included inthese photoelectric signals are canceled by the above subtractionoperation. Therefore, the substantial S/N ratio of the signal I_(mn) canbe increased.

It is preferable that at least on-axial chromatic aberrations of thevarious chromatic aberrations be corrected for the objective lens 22shown in FIG. 37, 42, or 46 to some extent. If the bandwidth ofwavelengths λ₁, λ₂, and λ₃ to be used is 100 nm or less, such anon-axial chromatic aberration can be corrected to some extent byselecting proper materials for a plurality of lens elements constitutingthe objective lens 22 or combining lens elements having differentrefractive indices and different dispersion ratios. This chromaticaberration need not be perfectly corrected in the objective lens 22. Thechromatic aberration can be corrected by the adjustment optical systems14, 16, and 18 shown in FIG. 37.

The 15th to 20th embodiments have been described above. In detecting thegrating mark MG on the wafer W or the fiducial mark plate FG inaccordance with the homodyne scheme, the grating mark MG must beprescanned in the pitch direction to sample the changes in levels of thephotoelectric signals. In this case, the simplest scheme is to change asignal waveform sampling clock signal C_(ps) shown in FIG. 38 or 43 intoa measurement pulse (e.g., one pulse every 0.02 μm) from aninterferometer 44 for measuring the position of the stage WST.

With this arrangement, waveform data of the respective photoelectricsignals generated during prescanning by several pitches of the gratingmarks MG are stored in a memory circuit 54 in correspondence with thepositions of the grating marks MG. Note that prescanning of the stageWST must be repeated a predetermined number of times corresponding tothe switching of the incident beams ±LF in units of wavelengths.

In the scheme for irradiating the two incident beams ±LF onto thegrating mark MG, the incident beams ±LF are preferably incident on thegrating mark MG at symmetrical angles in at least the pitch direction ofthe grating mark MG. In the scheme for projecting one incident beam onthe grating mark MG, as shown in FIG. 41, the incident angle of the beamis preferable zero (vertical incidence) with respect to the pitchdirection of the grating mark MG. That is, this indicates that theincident beams may be inclined in a direction (non-measurementdirection) perpendicular to the pitch direction of the grating mark MG.

In projecting illumination light beams switched in units of wavelengthsonto the measurement grating mark MG (or the fiducial mark), theplurality of laser beams of the respective wavelengths need not becoaxially synthesized, as shown in FIGS. 40, 41, and 37, but may beseparated in the non-measurement direction perpendicular to themeasurement direction (pitch direction) of the mark position and may beincident separately on the Fourier transform plane of the grating markMG. That is, the incident angles of the plurality of illumination beamson the grating mark MG may be different in the non-measurement directionin units of wavelengths of the illumination beams. The arrangement forthis is the same as that of FIGS. 35 and 36.

To obtain an incident beam, light from a halogen lamp or ahigh-luminance LED may be used in place of light from a laser lightsource. When light from a halogen lamp is used, a plurality ofwavelength selection filters (or interference filters) having a narrowbandwidth in different wavelength portions are. interchangeablyarranged, and light selected by time-divisionally switching thesefilters is guided through an optical fiber or the like and used. In thiscase, an incident beam for irradiating the grating mark MG on the waferhas a spectral intensity distribution continuous within the selectedwavelength bandwidth. For this reason, an interference filter(bandwidth: 3 nm to 10 nm) for extracting only a specific wavelengthcomponent may be fixedly or replaceably arranged in front of eachphotoelectric element in the light reception system.

As described above, in each of the 15th to 20th embodiments, a positiondetection illumination beam is switched in units of a plurality ofwavelengths, and diffracted light components generated from a positiondetection grating mark on a substrate are independentlyphotoelectrically detected in units of wavelength components. Markposition information is detected for each resultant photoelectricsignal, and the pieces of mark position information are averaged.High-precision position detection can be performed almost free from theinfluences of asymmetry of the marks and irregularities of the thicknessof the resist layer. The photoelectric signals independent in units ofwavelength components can be obtained in photoelectric detection of thediffracted light components from the mark. For this reason, even if theintensities of the illumination light beams are different in units ofwavelength components, the averaging effect using a multi-wavelengthbeam will not be advantageously impaired.

In each of the 15th to 20th embodiments described above, assume thatdiffracted light components to be photoelectrically detected arecomponents of higher order. In this case, when these multi-wavelengthcomponents (e.g., an interference beam of the 0th- and 2nd-orderdiffracted beams) are simultaneously received by a single photoelectricelement, a canceling phenomenon can be eliminated. Therefore,higher-precision position detection and alignment can be achieved.

In addition, the attenuation ratios (amplitude ratios) of the intensitylevels of the photoelectrically detected diffracted light components ofthe respective wavelengths are obtained. As for diffracted lightcomponents whose attenuation ratios are small and signal amplitudes arerelatively large, position detection is performed using weighted meancalculation. Therefore, higher-precision position detection than that ofsimple averaging can be obtained.

A twenty-first embodiment of the present invention will now bedescribed.

Generally a mark for alignment and position measurement formed on thesurface of a wafer or the like is made with a minute step difference onthe surface thereof, and has more or less asymmetry due to the waferprocess such as etching and sputtering in the semiconductor workingsteps, or the application irregularity of a photoresist layer, and itresults in reduced accuracy during mark position detection.

In an interference type alignment method of photoelectrically detectingthe mutual interference light of two diffracted lights created from agrating mark, and utilizing the photoelectric signal, the asymmetry ofthe grating mark becomes the asymmetry of the amplitude reflectance ofthe mark itself and acts to deteriorate position detection accuracy.That is, when the depth or the like of the groove bottom portions oflines constituting the grating mark has a difference in the gratingpitch direction or there is a partial difference in the thickness of theresist layer, the absolute value and phase of the amplitude reflectanceof the mark itself become asymmetrical in conformity with a variation inthe depth of the groove bottom portions and the thickness of the resistlayer.

As a result, the diffracted lights created from the grating mark becomedifferent in intensity and phase, for example, between positive ordercreated in the rightward direction relative to 0-order light andnegative order created in the leftward direction. Of these differences,the difference in intensity scarcely contributes to the deterioration ofposition detection accuracy, while a variation in phase greatly affectsposition detection accuracy.

So, the result of the simulation of position detection accuracy whichposes a problem in the heterodyne system using illuminating light of asingle wavelength will first be described with reference to FIGS. 1 and2. This simulation is such that supposing a case where coherentilluminating beams having a predetermined frequency differencetherebetween are applied from symmetrical two directions to a gratingmark on a wafer covered with a resist layer, the states (amplitude,phase, etc.) of the mutual interference light, i.e., interference beatlight, of 1st-order diffracted lights vertically created from thegrating mark have been found with the wavelength thereof varied.

FIG. 2 schematically represents a fragmentary enlarged cross-section ofa one-dimensional grating MG such as the wafer supposed in thesimulation and a resist layer PR applied to the surface thereof. Here,the pitch Pmg of the grating MG is set to 8 μm, the duty thereof is setto 50% (1:1), the level difference (or depth) of the groove is set to0.7 μm, and in the bottom portion of the grating MG, asymmetry of 0.1%is set as the taper (inclination) ΔS in the pitch direction.

The resist layer PR covering such a grating MG is assumed such that thethickness T1 of the-top portion of the grating MG from the surfacethereof is 0.9 μm and the depression amount ΔT in the surface of theresist layer which corresponds to the position of each bottom portion ofthe grating MG is ΔT=0.3T2 (0.21 μm). Such grating structure of FIG. 2is called a grating of which the amplitude reflectance is asymmetrical.

Now, FIG. 1 is a graph in which the wavelength λ (μm) of illuminatinglight or the interference light of +1st-order diffracted lights isplotted as the axis of abscissa and the relative amplitude of the change(AC component) of a signal conforming to the variation in the quantityof the interference light and the error amount (μm) of positiondetection are plotted as the axis of ordinates. In the result of thesimulation of FIG. 1, the conditions of the grating mark structure andresist layer of FIG. 2 have been set so that the wavelength λ in whichthe AC component of a photoelectric signal conforming to theinterference light received by the heterodyne system becomes just zero,i.e., a DC component alone, may be adjusted to the wavelength 0.663 μmof an He—Ne laser.

As is apparent from this, when a laser beam of wavelength 0.663 μm isused, it is seen that the detection error of the mark position becomesvery great in the vicinity (about ±20 nm) of that wavelength. This is amatter of course in the heterodyne system, and is because phasedifference measurement itself will become impossible if an AC componentconforming to a beat frequency is not included at all in thephotoelectric signal of which the phase difference is to be measured.This also holds quite true when position detection is effected by thehomodyne system under a grating mark structure and resist layer of thesame conditions.

So, it becomes effective not to detect only the interference light oftwo 1st-order diffracted lights travelling in a particular direction asin the simulation of FIGS. 1 and 2, but to photoelectrically detect theinterference light of 0-order light and −2nd-order diffracted lighttravelling in discrete directions, and take the mark position determinedon the basis of the signal also into account.

FIG. 47 shows the creation of 0-order light, +1st-order, ±2nd-order and±3rd-order diffracted beams when two irradiating beams ±LFs ofwavelength λ1 are applied to a diffraction grating mark MG atsymmetrical angles of incidence to thereby provide a beam incidencecondition under which interference fringes having an intensitydistribution of pitch Pif are produced and in addition, the pitch Pmg ofthe grating mark MG is brought into the relation that Pmg=2Pif. Whenthis pitch relation is satisfied, there is created the interference beamBM of two 1st-order diffracted beams +D1 b and −D1 a travellingvertically from the grating mark MG, as shown in FIG. 47.

In the diffracted beams ±Dna and ±Dnb shown in FIG. 47, the firstsuffixes 0, 1, 2 and 3 represents diffraction orders, the second suffixa means the diffracted beam created from the grating mark MG by theapplication of the illuminating beam +LFs, and the suffix b means thediffracted beam created from the grating mark MG by the application ofthe illuminating beam −LFs. Also, as regards the positive and negativesigns of the 1st-order and higher order diffracted beams in FIG. 47, thediffraoted beams turning clockwisely relative to the 0-order diffractedbeams D0 a and D0 b are negative, and the diffracted beams turningcounter-clockwisely are positive.

Now, the 2nd-order diffracted beam D2 a created by the application ofthe irradiating beam +LFs travels in a direction going back in theoptical path of the irradiating beam +LFs, and interferes with the0-order diffracted beam (regularly-reflected light) D0 b of theirradiating beam −LFs. Likewise, the 2nd-order diffracted beam +D2 bcreated by the application of the irradiating beam −LFs travels in adirection going back in the optical path of the irradiating beam −LFs,and interferes with the 0-order diffracted beam (regularly reflectedlight) D0 a of the irradiating beam +LFs. The interference beam of these0-order light and 2nd-order light, like the interference beam BM of±1st-order diffracted lights, varies in intensity in conformity with therelative displacement of the grating mark MG and the interferencefringes.

So, for example, the interference beam BM of 1st-order components(1st-order lights +D1 b, −D1 a) is photoelectrically detected to therebyfind the position (or the position offset) of the mark and also, theinterference lights of two sets of 2nd-order components (the pair of0-order light D0 b and 2nd-order light −D2 a, and the pair of 0-orderlight D0 a and 2nd-order light +D2 b) are photoelectrically detected,and the average value of mark positions individually found by the use ofthe signals of the two sets of 2nd-order components is found as theposition of the mark. It becomes effective to weight-average (weightedmean) the mark position detected by the use of the 1st-order componentsand the mark position detected by the use of the 2nd-order components,in conformity with the magnitude relation between the amplitude value ofthe signal of the 1st-order components and the average value of theamplitudes of the signals of the 2nd-order components.

The reason why the orders of the diffracted lights used for thedetection of the mark are thus changed is that the directions of thediffracted lights created from the grating MG differ in conformity withthe orders and therefore, even when the amplitude of the change in theintensity of the interference light of an order component travelling ina certain direction becomes small and detection accuracy is aggravated,the amplitude of the change in the intensity of the interference lightof an order component travelling in another direction does not become sosmall and detection accuracy is sometimes not aggravated.

This is ascertained from the result of a simulation shown in FIGS. 48Aand 48B. FIGS. 48A and 48B are graphs in which an He—Ne laser ofwavelength 0.633 μm is an irradiating beam and the relation between theamplitude of a change (AC component) of a signal with the stepdifference T2 of the grating MG in FIG. 2 as a parameter and theposition detection error is simulated. (Here, the pitch Pmg=8 μm, theduty is 1:1, the taper amount ΔS=0.1% is unchanged, and the thickness T1of the resist layer PR on the top surface of the grating is 1.15 μm.)

FIG. 48A shows the simulation in the case of the interference beam BM of1st-order components (1st-order lights ±D₁₁), and FIG. 48B shows thesimulation in the case of the interference light of 2nd-order components(0-order lights ±D01 and 2nd-order diffracted lights ±D21).

As will be seen from FIGS. 48A and 48B, the amplitude components ofsignals obtained by photoelectrically detecting the interference lightsof 1st-order components and 2nd-order components change greatly inconformity with a delicate change in the step difference (T2) of thegrating mark. For example, when in FIG. 48A, the step difference T2 ofthe grating mark is in the range α of about 1.03-1.13 μm and in therange β of about 1.25-1.32 μm, the amplitude A01 of the change in theintensity of the interference light of 1st-order components generallybecomes small and as a result, the position detection error ΔX01 becomesextremely great or exhibits a considerably unstable change tendency.

However, when portions in FIG. 48B in which the step difference T2 ofthe mark corresponds to the ranges α and β of FIG. 48A are examined, itis seen that the amplitude A02 of the change in the intensity of theinterference light of 2nd-order components is relatively great and thedeterioration of the position detection error ΔX02 is little. While theamplitudes of the changes in the signals in FIGS. 48A and 48B are bothrepresented as relative values, the scales thereof are combined togetherin FIGS. 48A and 48B.

In the conventional alignment system of the two-beam interference typedisclosed, for example, in the publication (H), from the tendency asdescribed above, the position offset measurement by the interferencelight of 2nd-order components has been merely selected when theintensity amplitude of the interference light of 1st-order components issmall.

Now, the influence of the asymmetry of the optical amplitude reflectanceof the grating mark including the resist layer remarkably appears ineach discrete order component of the basic period (1st-order diffractedlight) component, double period (2nd-order diffracted light) component,triple period (3rd-order diffracted light) component, . . . , of thedetected light. So, examining the actual grating mark, it has been thereality that no strong correlation is seen among the influences(particularly phase transition, etc.) of the asymmetry included in eachdetected light of each discrete order component.

When for example, there is predetermined asymmetry in the amplitudereflectance of the grating mark to be detected and the measurement ofthe mark position is effected on the basis of the photoelectricdetection of the interference light of mth-order components (e.g.±1st-order diffracted lights), if the measured position includes apositive (+) position error relative to a true measured value, and ifthe measurement of the mark position is effected on the basis of thephotoelectric detection of the interference light of nth-ordercomponents (e.g. 0-order and 2nd-order), the measured value may includea positive (+) position error or may include a negative (−) positionerror.

Thus, there is little or no correlativity between the irregularity ofthe error included in the position measurement (mth-order componentmeasured value) based on the interference light of mth-order componentsand the irregularity of the error included in the position measurement(nth-order component measured value) based on the interference light ofnth-order components. However, statistically examining, it has beenfound that as compared with the irregularity of the error of only themth-order component measured value and the irregularity of the error ofonly the nth-order component measured value, the irregularity of theerror of the average value of the mth-order component measured value andthe nth-order component measured value becomes smaller corresponding tothe averaging effect.

Further, if in the averaging of the mth-order component measured valueand the nth-order component measured value, use is made of not a simpleaverage, but weighted mean using a weight coefficient conforming to eachof the amplitude information of a photoelectric signal obtained byphotoelectrically detecting the interference light of mth-ordercomponents (e.g. the ratio between an ideal signal amplitude valueobtained by the grating or the like of a fiducial plate and a signalamplitude value obtained from a grating which may include asymmetry on awafer) and the amplitude information of a photoelectric signal obtainedby photoelectrically detecting the interference light of nth-ordercomponents (e.g. the ratio between the ideal signal amplitude value anda signal amplitude value obtained from the actual grating), it becomespossible to reduce the bad influence when the amplitude of thephotoelectric signal of a certain order component has become extremelysmall.

Also, the diffracted light of each order becomes smaller in intensity interms of principle as the order becomes higher and therefore, theamplitude of the photoelectric signal of the interference light by eachorder component also becomes smaller in conformity with the highness ofthe order. So, theoretical weighting conforming to the order is furtheradded to provide a weighted mean, whereby more highly accurate positiondetection becomes possible.

So, each embodiment of the present invention constructed in accordancewith the above-described principle will hereinafter be described withreference to the drawings. First, FIG. 49 shows the construction of aposition detecting apparatus according to a twenty-first embodiment ofthe present invention, and it is to be understood here that the positiondetection of a semiconductor wafer W having a diffraction grating markMG formed unevenly on the surface thereof by etching is effected.Further, the present embodiment adopts an alignment method of theheterodyne type which applies two coherent beams having a predeterminedfrequency difference therebetween from two symmetrical directions to thegrating mark on the wafer to thereby produce on the grating mark aone-dimensional interference fringe (moving at a speed conforming to thebeat frequency) by the interference between the two beams.

Now, an irradiating beam LB (of one wavelength λ within a range of theorder of wavelength band 500-1000 nm) from a light source LS such am anHe—Ne laser or a semiconductor laser as a coherent light source isconverted into two illuminating beams +LFs and −LFs intersecting eachother at a predetermined angle by an optical system TBO for creating twobeams. This optical system TBO for creating two beams, as will bedescribed later in detail, provides a predetermined frequency difference(beat frequency) between the two illuminating beams +LFs and −LFs and isalso provided with a system for producing a reference signal for theheterodyne type.

Both of the illuminating beams ±LFs are parallel light beam and aretransmitted through a beam splitter BS1 and thereafter, arrives at anillumination field aperture plate AP. This illumination field apertureplate AP has a rectangular opening for prescribing an illuminating areato the grating mark MG on the wafer W, and is disposed on the surface ofintersection between the two illuminating beams ±LFs. This apertureplate AP is constructed by depositing vaporization low-reflectionchromium as a light intercepting layer on a transparent glass plate, andforming a rectangular transparent window (opening) in a portion of thechromium layer by etching.

The two illuminating beams ±LFs prescribed by the opening in this stopplate AP enters a first lens system (a fore unit lens system) G1 actingas a Fourier transform lens. These two illuminating beams ±LFs areconverged by the first lens system (fore unit lens system) G1 so as tobecome beam waist on a Fourier transform plate EP, and thereafterdiverge and enter a second lens-system (a rear unit lens system) G2.Thereafter, the two illuminating beams +LFs are converted into twoparallel light beams intersecting each other on the wafer by the secondlens system G2, and one-dimensional interference fringes are formed onthe wafer W.

Here, the plate of the rectangular opening in the aperture plate AP isdisposed at the front side focal length f11 of the first lens system G1,the Fourier transform plane EP is produced at the rear side focal lengthf12 of the first lens system G1, the front side focal length f21 of thesecond lens system G2 is disposed so as to coincide with the Fouriertransform plate EP, and the surface (grating mark MG) of the wafer W isdisposed so as to lie at the rear side focal length f22 of the secondlens system G2.

Accordingly, if the first lens system G1 and the second lens system G2are arranged along one and the same optical axis AX and the center ofthe rectangular opening in the aperture plate AP is disposed on theoptical axis AP, the principal rays of the two illuminating beams ±LFsbecome parallel to each other with the optical axis AX interposedtherebetween in the space near the first lens system G1 and the secondlens system G2.

Now, the pitch Pif of the interference fringes produced on the wafer Wby the two illuminating beams 'LFs is represented by Pif=λ/2 sin θw whenthe angles of incidence of the illuminating beams ±LFs onto the waferare defined as ±θw and the wavelength of the illuminating beams isdefined as λ. Here., the imaging optical system by the first lens systemG1 and the second lens system G2 makes the aperture plate AP and thewafer W conjugate with each other and therefore, when the angle ofincidence of each of the two illuminating beams ±LFs irradiating theaperture plate AP is defined as θr and the imaging magnification (theratio of the image size when the aperture plate AP side is seen from thewafer W side) of the imaging optical system by the first lens system G1and the second lens system G2 is defined as MP, there is the relationthat pM·sin θw=sin θw.

The periodicity direction of the grating mark MG is made coincident withthe X-axis direction (the left to right direction in the plane of thedrawing sheet of FIGS. 5A and 5B) of the coordinates system XY, and theperiodicity direction of the interference fringes produced bysymmetrically inclining the principal rays of the two illuminating beams±LFs arriving at the wafer W in a plane prescribed by the X-axis and theoptical axis AX (the Z axis) is made coincident with the X-axisdirection. Further, the angle of incidence θw (or θr) of theilluminating beams ±LFs is set so that the pitch Pif of the interferencefringes and the pitch Pmg of the grating mark MG may assume the relationthat 2Pif=Pmg.

When such conditions are provided two 1st-order diffracted lights +D1 band −D1 a travelling in a vertical direction (Z-direction) from thegrating mark MG as previously described are coaxially created and these1st-order diffracted lights +D1 b and −D1 a become an interference beamBM resulting from the interference therebetween, and this interferencebeam BM passes through the second lens system G2, the first lens systemG1 and the opening in the aperture plate AP to a beam splitter BS1,where it is reflected substantially at a right angle and is condensed ona photoelectric element DT1 disposed substantially conjugately with theFourier transform plane EP, by a lens system G3. This photoelectricelement DT1 outputs a photoelectric signal Im1 conforming to a variationin the light intensity of the interference beam BM as long as thegrating mark MG is situated in the irradiation area of the illuminatingbeams ±LFs, and outputs a predetermined level of almost zero when thegrating mark MG is not present in the irradiation area.

When the relative positional relation regarding the sine wave-likeintensity distribution of the interference fringes produced by theilluminating beams +LFs and the pitch direction (X-direction) of thegrating mark MG uniformly changes with time, the photoelectric signalIm1 level-changes into a sine wave-like form, and if the relativepositional relation between the intensity distribution of theinterference fringes and the grating mark MG is stationary, the level ofthe photoelectric signal Im1 will be maintained at a certainpredetermined level in the sine wave-like level change. In order tocause the photoelectric signal Im1 to thus level-change into a sinewave-like form, in the homodyne system, the moving stage WST for holdingand positioning the wafer W is scanned and moved in X-direction.

The present embodiment, however, adopts the heterodyne system in which apredetermined frequency difference is provided between the twoilluminating beams ±LPs and therefore, even in a positional relationwherein the irradiation area by the illuminating beams ±LFs and thegrating mark MG are stationary, the photoelectric signal I_(m1) becomesan AC waveform which level-change into a sine wave-like form at a beatfrequency.

On the other hand, as described in connection with FIG. 47, theinterference beam Bm02 of the regularly reflected light (0-order light)and the −2nd-order diffracted light of the illuminating beam +LFscreated from the grating mark MG of the wafer W goes back along theoptical path of the illuminating beam −LFs and enters the second lenssystem G2, whereafter it arrives at the Fourier transform plane EP andfurther passes through the first lens system G1 and the opening in theaperture plate AP to the beam splitter BS1. Likewise, the interferencebeam Bm20 of the regularly reflected light (0-order light) and the+2nd-order diffracted light of the illuminating beam +LFs created fromthe grating mark MG goes back along the optical path of the illuminatingbeam +LFs and passes through the second lens system G2, the first lenssystem G1 and the opening in the aperture plate AP to the beam splitterBS1.

The interference beams BmO2 and Bm20 reflected by the beam splitter BS1become parallel light beams of different angles and enter the lenssystem G3, and arrive at photoelectric elements DT2 a and DT2 b havinglight receiving surfaces disposed substantially conjugately with theFourier transform plane EP. The photoelectric signals Im02 and Im20 ofthese photoelectric elements DT2 a and DT2 b, respectively, like thephotoelectric signal Im1 obtained from the interference beam BM,level-change in conformity with a change in the relative position of theintensity distribution of the interference fringes and the grating markMG.

However, the signals Im02 and Im20 as 2nd-order components, unlike thesignal Im1 as a 1st-order component, assume a DC level conforming to thesimple reflectance of the surface of the wafer because the 0-orderlights of the illuminating beams ±LFs are present even when the gratingmark MG is not present in the irradiation area of the illuminating beams±LFs.

Now, the stage WST holding the wafer W thereon is two-dimensionallymoved in X-direction or Y-direction by a driving system 42 comprising acombination of a motor and a feed screw or a combination of a linearmotor and an air guide. Further, the two-dimensional moved position(coordinates position) and the amount of movement of the stage WST aresequentially measured by a laser interferometer 44 for projecting alaser beam onto a reflecting mirror 43 fixed to a portion of the stageWST. By the cooperation of the driving system 42 and the laserinterferometer 44, the stage WST is position-controlled with theaccuracy of the order of the resolving power of the interferometer 44 tothereby effect the prealignment (within ±¼ of the pitch Pmg of thegrating mark MG) of the grating mark MG into the irradiation area of theilluminating beams ±LFs, the scanning movement of the grating mark MGrelative to the irradiation area of the illuminating beams ±LFs, or thepositioning of a desired portion (such as a shot area) on the wafer Wrelative to a reference point.

Further, a fiducial mark plate FG formed with a fiducial grating mark ofa pitch similar to that of the grating mark MG on the wafer W isprovided on a portion of the stage WST. On this mark plate FG, astrength grating of the reflection type in which line and space arepatterned with a chromium layer on the surface of quarty glass is formedas a fiducial grating mark. This strength grating, unlike a phasegrating such as the grating mark MG unevenly formed on the wafer W, hasthe feature that it has not asymmetry and its diffraction efficiencydoes not depend on the wavelength of the illuminating light (ordetecting light), that is, its amplitude reflectance is free ofasymmetry. Further, the reflectance of the chromium layer hardly changesin the wavelength band (generally 0.5-0.8 μm) of the illuminating lightused for position detection.

In the construction of the optical system as described above, an exampleof the construction of the optical system TBO for creating two beams fordetecting the position or the position offset error of the grating mark(or the fiducial grating mark of the fiducial mark plate FG) will now bedescribed with reference to FIGS. 50 and 51. FIG. 50 shows the plandisposition of the optical system for creating two beams, and it is tobe understood here that it uses two acousto-optical modulators (AOMs)100A and 100B.

A beam LB from a laser beam source LS passes through a prism 101 forminutely deflecting a beam optical path, whereafter it is reflected by amirror 102 and is divided into two beams by a beam splitter 103. Thebeam transmitted through this beam splitter 103 enters the AOM 100A at apredetermined angle, and the beam reflected by the beam splitter 103enters the AOM 100B at a predetermined angle. These two AOMs 100A and100B are driven by frequencies fa and fb, respectively, of a highfrequency band (50-100 MHz), and the frequency fa is set to e.g. 80.050MHz and the frequency fb is set to e.g. 80.000 MHz. The differencefrequency (fa−fb=50 KHz) between these two frequencies fa and fb becomesthe beat frequency in the heterodyne system. While the two AOMs 100A and100B are provided in parallel here, the two AOMs may be disposed intandem (in series), and a beam may be caused to enter the first-stageAOM under the diffraction condition of Raman-Nath, and two 1st-orderdiffracted beams produced there may be caused to enter the second-stageAOM under the condition of Bragg diffraction to thereby create two beamsof a desired frequency difference.

Now, a diffracted beam and 0-order beam diffracted by a diffractiongrating in the AOM 100A are reflected by a mirror 104 and enter a lens105, whereafter the 0-order beam is intercepted by a stop 106 and thediffracted beam alone passes through plane parallel glass 107 andarrives at a composite beam splitter 115. Likewise, a diffracted beamand 0-order beam diffracted by a diffraction grating in the AOM 100B arereflected by a mirror 108 and enter a lens 109, whereafter the 0-orderbeam is intercepted by a stop 110 and the diffracted beam alone passesthrough plane parallel glass 111 and arrives at the composite beamsplitter 115.

This composite beam splitter 115 transmits therethrough the diffractedbeam from the AOM 100A as an illuminating beam ±LFs and totally reflectsthe diffracted beam from the AOM 100B as an illuminating beam −LFs.These two illuminating beams ±LFs are combined so as to become parallelto each other with a predetermined spacing therebetween as shown afterthey have passed through the composite beam splitter 115, and thespacing is adjusted by changing the angles of inclination of the planeparallel glass 107, 111.

Now, the two illuminating beams ±LFs having emerged from the compositebeam splitter 115 enter a beam splitter 117 of usual amplitude divisionas shown in FIG. 51 which is a view of the optical construction of FIG.50 as it is seen from sideways thereof, and are divided into two pairsof beams thereby. Each of a pair of beams reflected by the beam splitter117 is converted into parallel light beams intersecting each other at apredetermined angle by a lens 118, and becomes a pair of illuminatingbeams ±LFs, which travel toward the aperture plate AP in FIG. 49.

On the other hand, each of a pair of beams transmitted through the beamsplitter 117 is converted into parallel light beams intersecting eachother at a predetermined angle on a reference grating plate SG by theaction of a lens 119, as shown in FIG. 50. This reference grating plateSG is formed with an amplitude grating of the transmission type, and thepitch of the reference grating thereof is set to double the pitch of theintensity distribution of interference fringes formed by theintersection between the two beams projected from the lens 119.Therefore, +1st-order diffracted beam and −1st-order diffracted beam arecoaxially created in a vertical direction from the reference gratingplate SG, and they become interference beams interfering with each otherand arrive at a space filter 38.

This space filter 38 has an opening window for intercepting two 0-orderlights (broken lines in FIG. 50) travelling obliquely from the referencegrating plate SG and transmitting therethrough the interference beam of+1st-order diffracted beams travelling perpendicularly from thereference grating plate SG, and the interference beam is received by aphotoelectric element 40 provided to output a reference signal Ims inthe heterodyne system.

In the above-described construction, the condensing point (beam waist)of each beam condensed by the lenses 105 and 109 is set on a particularplane near the composite beam splitter 115, and the particular plane isdetermined substantially conjugately with the Fourier transform plane EPin FIG. 49. Accordingly, by adjusting the angles of inclination of theplane parallel glass 107, 111 in FIG. 50, it is possible to adjust theangle of intersection and the angles of incidence θr and θw of the twoilluminating beams ±LFs arriving at the aperture plate AP or the wafer W(the fiducial plate FG) in FIG. 49, or the telecentricity of theilluminating beams.

Also, the position and position offset amount of the grating mark MG onthe wafer W (the fiducial plate FG) can be found by measuring the phasedifferences among the signals Im1, Im02 and Im20 from the photoelectricelements DT1, DT2 a and DT2 b with the phase of the signal Ims from thephotoelectric element 40 in FIG. 50. At that time, in the presentembodiment, a weight coefficient is given to the position offset amount(or the phase difference) measured from each of 1st-order component and2nd-order component in conformity with the amplitude (peak to peak)values of the photoelectric signals Im1, Im02 and Im20 obtainedparticularly from the grating mark MG on the wafer W to thereby effectweighted mean, thus determining the position offset amount of thegrating mark MG on the wafer W.

So, an example of the signal processing circuit and position controlcircuit of the present embodiment will now be described with referenceto FIG. 52. In the case of the heterodyne system shown in FIGS. 49, 50and 51, the signals Im1, Im02, Im20 and Ims from the photoelectricelements DT1, DT2 a, DT2 b and 40 assume a sine wave-like AC waveform asshown in FIGS. 53A-53D as long as interference beams BM, Bm02 and Bm20are created from the grating mark MG on the wafer W or the fiducial markplate FG. The axes of abscissas of FIGS. 53A-53D represent time, and theaxes of ordinates represent the intensity level of each signal.

FIG. 53D represents a change with time in the intensity of the signalIms which is a reference signal, and FIGS. 53A to 53C show an example ofthe changes with time in the intensity of the signals Im1, Im02 and Im20when the interference beams BM, Bm02 and Bm20 from the grating mark MGon the wafer W are received. It is to be understood here that when thephase of the signal Ims is taken as the reference, the phase of thesignal ImI shifts by −Δψ11 relative to the reference phase point (timetr) of the signal Ims, the phase of the signal Im02 shifts by −Δψ02relative to the signal Ims, and the signal Im20 shifts by +Δψ20 relativeto the signal Ims. Also, it is to be understood that at the time, theamplitude (peak to peak of the AC component) of the signal Im1 is E11,the amplitude of the signal Im02 is E02 and the amplitude of the signalIm20 is E20.

Now, the absolute values of the phase shift amounts −Δψ02 and +Δψ20 ofthe photoelectric signals Im02 and Im20 obtained by photoelectricallydetecting the interference beams Bm02 and Bm20 of 0-order light and2nd-order light are generally great as compared with the absolute valueof the phase shift amount −Δψ11 of the photoelectric signal Im1 in thecase of the interference beam BM of ±1st-order diffracted lights. Theinterference beams Bm02 and Bm20 of 0-order light and 2nd-order lightare created at a symmetrical angle on the opposite sides of theinterference beam BM of 1st-order diffracted lights −D1 a and +D1 b, asdescribed in connection with FIG. 47, and therefore, the phase shiftamounts −Δψ02 and +Δψ20 thereof generally have the tendency of thesubstantially opposite direction relative to the reference phase (timetr).

Now, in the circuit blocks shown in FIG. 52, the signals Im1, Im02, Im20and Ims are inputted to an analog-digital converting (A/D converter)circuit unit 50, in which the intensity level of each signal at thatmoment is converted into a digital value is response to a clock signal(pulse) Cps from a sampling clock generating circuit 52.

The frequency of the clock signal Cps is determined sufficiently higherthan the beat frequencies (e.g. 50 KHz) of the signals Im and Ims, andthe clock signal Cps is also sent to a waveform memory circuit unit 54and is used for the renewal of a memory address when the digital value(data) from the A/D converter 50 is stored.

Accordingly, in the waveform memory circuit unit 54, four waveform datashown in FIGS. 53A to 53D are digital-sampled over predetermined periods(e.g. ten or more periods) of the signals Im1, Im02, Im20 and Ims. Atthis time, the four signals are sampled at a time by the common clocksignal Cps and therefore, it is to be understood that each waveform datain the waveform memory circuit unit 54 has no shift on the time axis.

Now, the waveform data in the memory circuit unit 54 are read into aphase difference and position offset detection circuit unit 56, in whichphase differences Δψ11, Δψ02 and Δψ20 as shown in FIGS. 53A to 53D arecalculated by digital calculation (Fourier integral method). If aspreviously assumed, the pitch Pmg of the grating mark MG on the wafer Wand the pitch Pif of the interference fringes applied thereonto are setto Pmg=2Pif, a period of each waveform shown in FIGS. 53A to 53Dcorresponds to Pmg/2.

Also, generally the measurement of phase difference is effected within arange of ±180 degrees and therefore, the detection circuit 56 calculatesthe position offset amounts ΔX11, ΔX02 and ΔX20 within a range of ±Pmg/4on the basis of the calculated phase differences Δψ11, Δψ02 and Δψ20, inaccordance with

ΔX 11=Pmg·Δψ11/4π

ΔX 02=Pmg·Δψ02/4π

ΔX 20=Pmg·Δψ20/4π.

These offset amounts ΔX11, ΔX02 and ΔX20 each represent the positionoffset of each order component of the grating mark MG relative to thereference grating SG.

Assuming here that about 0.2° is obtained as the resolving power ofphase difference measurement, the resolving power of the offset amountis nearly (0.2/180)·Pmg/4, and when the pitch Pmg is 4 μm, about 0.002μm (2 nm) is obtained as a practical range.

On the other hand, a signal amplitude and amplitude ratio detectioncircuit unit 58 reads out the waveform data of FIGS. 53A to 53D storedin the waveform memory circuit unit 54, and detects the amplitude valuesE11, E02 and E20 of the respective waveforms by digital calculation.

In this detection circuit unit 58, there are stored in advance theamplitude values A11, A02 and A20 of the photoelectric signals Im1, Im02and Im20 obtained when the interference beams created from the referencegrating mark on the fiducial mark plate FG are received by thephotoelectric elements DT1, DT2 a and DT2 b. That is, before the gratingmark MG on the wafer W is measured, the fiducial grating mark on thefiducial mark plate FG is moved to under the irradiation area of theilluminating beams ±LFs to thereby generate the signals as shown inFIGS. 53A to 53D from the photoelectric elements DT1, DT2 a and DT2 b,and these signals are stored in the waveform memory circuit unit 54,whereafter amplitude values A11, A02 and A20 are detected by theamplitude detection circuit 58 and are stored.

If at this time, the static position of the stage WST in which thefiducial mark plate FG is detected is read from the laser interferometer44 and is stored and also the position offset amounts ΔXb11, ΔXb02 andΔXb20 of every order are found by the offset amount detection circuitunit 56, they can be utilized as the data during the base linedetermination.

The base line referred to herein means a minute error amount occurringbetween the position offset amount ΔXb11 found from the interferencebeam of 1st-order components for the grating mark on the opticallystable fiducial plate FG and the average value ΔXb22=(ΔXb02+ΔXb20)/2 ofthe two position offset amounts ΔXb02 and ΔXb20 found from theinterference beam of 2nd-order components.

Originally, in the apparatus construction shown in FIG. 49, if the pitchPif of the interference fringes produced on the fiducial plate FG by theilluminating beam of a single wavelength strictly coincides with thepitch Pmg of the fiducial grating mark and the electrical responsivenessand strain characteristic of each photoelectric elements aresufficiently uniform, the values of the position offset amounts ΔXb11and ΔXb22 regarding the mark plate FG ought to completely coincide witheach other.

As a realistic problem, however, if the resolving power is as great asthe order of 2 nm, it is difficult to adjust the light sending systemand the detection system so that the position offset amounts ΔXb11 andΔXb22 may be uniform to the degree of the resolving power. Thus, thedifference between the position offset amounts ΔXb11 and ΔXb22 measuredby the mark plate FG remains as the offset (base line error) inherent tothe alignment system shown in FIG. 49.

The base line error is indirectly corrected by correcting andcalculating the mark position offset amount determined by the positionoffset amounts ΔX11, ΔX02 and ΔX20 found by the detection circuit 56 bydetecting the grating mark MG on the wafer W, by the minute error amountof the position offset amounts ΔXb11 and ΔXb22 previously found by thefiducial plate FG.

So, turning back to FIG. 52, the amplitude ratio =detection circuit unit58 has also the function of calculating the ratios K11, K02 and K20between the amplitude values A11, A02, A20 stored in advance bydetecting the fiducial plate FG in the above-described manner and theamplitude values E11, E02, E20 obtained when the grating mark MG on thewafer W is detected, as K11=E11/A11, K02=E02/A02 and K20=E3/A3. It isfor the ratios K11, K02 and K20 to be used as the weight coefficientduring the weighted mean of the position offset amount determined byeach of 1st-order component and 2nd-order component.

However, it is also desired to prepare a calculation mode in whichsimple weight coefficients conforming to the amplitude values E11, E02and E20 can be used in the calculation of the weighted means. Which setof weight coefficients should preferably adopted in terms of accuracydepends greatly on the asymmetry of the amplitude reflectance of thegrating mark MG on the wafer and therefore, it is desirable that designbe made such that any set of weight coefficients can be suitablyselected. So, whichever of the set of simple weight coefficientsconforming to the amplitude values E11, E02 and E20 and the set ofweight coefficients conforming to the ratios K11, K02 and K20 found bythe use of the fiducial mark plate FG is to be selected, those two setsof weight coefficients are typically represented by C11, C02 and C20,respectively.

Now, the data of the position offset amounts ΔX11, ΔX02, ΔX20 and theweight coefficients C11, C02, C20 found in the above-described mannerare sent to a weighted mean calculation circuit unit 60, in which theoffset amount ΔX of the grating mark MG having the weight added theretois calculated. However, as regards the position offset amounts ΔX02 andΔX20 found from the interference beams Bm02 and Bm20 of 2nd-ordercomponents, it is necessary in principle to effect weighted mean tothereby find a position offset amount ΔX22 of the grating mark MGdetected by the 2nd-order components alone.

So, the weighted mean calculation circuit unit 60 first calculates theposition offset amount ΔX22 determined by the 2nd-order componentsalone, on the basis of the following expression:

ΔX 22=(ΔX 02+ΔX 20)/2

Likewise, the weight coefficients C02 and C20 conforming to theamplitude values E02 and E20 of the interference beams Bm02 and Bm2O of2nd-order components (or the ratios K02 and K20 found by the use of thefiducial mark plate FG) are averaged, and a weight coefficient C22regarding the position offset amount ΔX22 determined by 2nd-ordercomponents alone is calculated on the basis of the following expression:

C 22=(C 02+C 20)/2

Thus, the position offset amounts ΔX11, ΔX22 and the weight coefficientsC11, C22 by the 1st-order component and the 2nd-order component,respectively, are determined and therefore, the weighted meanscalculation circuit unit 60 calculates the final position offset amountΔX of the grating mark MG by the following calculation:

ΔX=(C 11·ΔX 11+C 22·ΔX 22)/(C 11+C 22)

Thereby, the position detection error included in the offset amount ΔX11determined on the basis of 1st-order component alone and the positiondetection error included in the offset amount ΔX22 determined on thebasis of 2nd-order component alone are averaged and the irregularity ofthe errors can be made small by the averaging effect.

Also, when one of the degree of modulation of the interference beam Bmof 1st-order components (the amplitude value of the signal Im1) and thedegrees of modulation of the interference beams Bm02 and Bm20 of2nd-order components (the amplitude values of Im02 and Im20) becomesextremely small, the position detection accuracy by the order componentsmay sometimes be extremely aggravated. In the present embodiment,however, the weight coefficients C11, C02 and C20 obtained from thesignals Im1, Im02 and Im20, respectively, are utilized for makingweighted means and therefore, the weight scarcely acts on the result ofthe position offset detection by the order component reduced inamplitude, and the bad influence (accuracy deterioration) on theposition offset amount ΔX finally found is reduced.

Now, the weight coefficients C11 and C22 utilized when theabove-described position offset amount ΔX is found are ones dependingsimply on the amplitude values of the signals Im1, Im02 and Im20, butsecond weight coefficients conforming to the order components to bephotoelectrically detected may be added to thereby calculate the finalposition offset amount ΔX. Specifically, a second weight coefficient tothe signal Im1 of 1st-order component is defined as B11 and a secondweight coefficient to the signals Im02 and Im20 of 2nd-order componentsis defined as B22, and the position offset amount ΔX is calculated bythe following expression:

ΔX=(C 11·B 11·ΔX 11+C 22·B 22·ΔX 22)/(C 11·B 11+C 22·B 22)

Accordingly to the weighted mean having these second weight coefficientsadded thereto, any reduction in the position detection accuracy by thetheoretical difference between the degrees of modulation (the amplitudesof the signals) of the interference beams BM, Bm02 and Bm20 due to thedifference between the orders of the diffracted beams created from thegrating mark MG can be prevented and more highly accurate positiondetection becomes possible. This is attributable to the phenomenon that±1st-order diffracted lights are greater in intensity than ±2nd-orderdiffracted lights.

As regards the numerical value examples of the second weightcoefficients B11 and B22, according to the result of the experiment, thebest accuracy has been obtained when they are in a range of the order ofB22/B11=1-0.25. However, when B22/B11=1, the second weight coefficientssubstantially do not work, and when B22/B11=0.25, the weight to theresult of position detection (the position offset amount ΔX22) by2nd-order component is most emphasized. This ratio B22/B11 of the secondweight coefficients may be stepwise changed over and set in conformitywith the processes (such as etching, evaporation and diffusion) appliedto the wafer as an example.

The offset amount ΔX found in the above-described manner is the offsetof the grating mark MG in the pitch direction relative to the referencegrating SG, and the data thereof is sent to a position control anddisplay 62 shown in FIG. 52 and is also sent to a servo control circuitunit 64 when the wafer is aligned on real time.

This servo control circuit unit 64 has two functions, one of which isthe function of feedback-controlling the driving system 42 until theoffset amount ΔX reaches a predetermined value (direct servo mode). Inthe case of this function, the operations of the A/D converter circuit50, the memory circuit unit 54, the offset amount detection circuit unit56 and the average calculation circuit unit 60 are successivelyrepeated, whereby the value of the offset amount ΔX is calculated withineach very short time (e.g. several msec.).

In that case, the calculation of the weight coefficients C11, C02 andC20 by the amplitude ratio detection unit 58 may be effected only thefirst one time or may be effected each time the calculation of theoffset amount ΔX is effected. Of course, when the calculation of thecoefficients C11, C02 and C20 is effected each time, the values of thecoefficients C11, C02 and C20 may somewhat change each time the offsetamount ΔX is calculated by the weighted mean circuit unit 60. Also, whenthe calculation of the coefficients C11, C02 and C20 is effected at thefirst one time or a plurality of times, the same values of thecoefficients are used as long as the same grating mark MG is detectedthereafter.

On the other hand, the other function of the servo control circuit unit64 is the function of moving the wafer stage WST on the basis of themeasured value by the laser interferometer 44 (interferometer servomode). This function is used when for example, the grating of thefiducial mark plate FG on the stage WST or the grating mark MG on thewafer W is positioned in the irradiation area of the illuminating beams±LFs or when with the detected position of the grating mark MG as thereference, any point (for example, other grating mark) on the wafer W ispositioned just beneath the second lens system G (the irradiation areaof the illuminating beams LFs).

In the case of this interferometer servo mode, the target positioninformation of the wafer stage WST is outputted from the positioncontroller 62 to the servo control circuit unit 64, which thusfeedback-controls the driving system 42 so that the difference betweenthe current position data of the stage WST read from the counter in thelaser interferometer 44 at every several tens of ps and the targetposition data may fall within a predetermined allowable range (e.g.+0.04 μm).

When the direct servo mode is to be executed in subsequence to theinterferometer servo mode, the possible range by the direct mode is±Pmg/4 relative to the pitch Pmg of the grating mark MG. This is becauseif the offset is greater than that, the positioning will be done whilethe offset corresponding to a half of one pitch of the grating mark MGremains created.

So, instead of the allowable range of the positioning of the stage WSTduring the interferometer servo mode being steadily limited to ±0.04 μm,the allowable range may be changed over to ±((Pmg/4)−α) only when thegrating mark MG (or the fiducial mark plate FG) is detected. Here, α isα<(Pmg/4).

When for example, the pitch Pmg is 4 μm, if the allowable range is ofthe order of ±0.5 μm (α=0.5 μm), positioning servo is possible withaccuracy much looser than the ordinary allowable range (±0.04 μm) andthus, the run-in time is shortened. When the loose allowable range (±0.5μm) has been entered, the mode is immediately changed over to the directservo mode, whereby high-speed and highly accurate positioning(alignment) becomes possible.

Now, the position control and display 62 has, besides theabove-described instruction of the changeover of the servo mode, thefunction of displaying the coordinates position and found offset amountΔX of the grating mark MG. Also, in some cases, it stores and preservesthe values of the ratios C11, C02 and C20 which are the weightcoefficients when the grating mark MG has been detected. In this case,when grating marks MG of the same shape are formed at a number oflocations on the wafer W and the positions of those marks MG are to besuccessively detected, if the coefficients C11, C02 and C20 are alsomemorized, it will become possible to verify in which mark MG on thewafer W there has been a problem attributable to the asymmetry or theirregularity of the resist layer.

Design may also be made such that any portion on the wafer W in whichthe weight coefficients (C11, C02, C20) have greatly changed isgraphically displayed. In this case, if the wafer before a resist layeris applied thereto via chemical processes such as the diffusing step andthe etching step is mounted on the apparatus of FIG. 49 and the changein the weight coefficients is found, the influence of the chemicalprocesses on the surface of the wafer can also be indirectly examined.Further, if a resist layer is applied to the wafer and likewise anychange in the weight coefficients is found and is compared with thechange in the weight coefficients before the application of the resistlayer, the influence of the resist layer can also be indirectlyexamined.

In the above-described twenty-first embodiment, the fiducial mark plateFG is provided on the stage WST and provision is made of the function offinding the rates of change of the signal amplitude of each orderobtained from the grating mark free of asymmetry and the signalamplitude of each order obtained from the grating mark having asymmetry,i.e., weight coefficients K11, K02 and K20, by the use of the fiducialmark plate FG, and therefore, position detection higher in reliabilityand reproducibility than in a case where the weight coefficients aresimply determined by only the magnitudes of the amplitudes of thephotoelectric signals obtained by the photoelectric detection of theinterference beam of each order component becomes possible.

The construction of a position detecting apparatus according to atwenty-second embodiment of the present invention will now be describedwith reference to FIGS. 54 and 55. In this embodiment, unlike theheterodyne system in the previous twenty-first embodiment, a positiondetecting system of the homodyne type is applied, and this is applied asa system for photoelectrically detecting the interference light of±1st-order diffracted lights created from a diffraction grating patternformed on a substrate to be position detected, and the interferencelight of ±2nd-order diffracted lights created from the diffractiongrating pattern on the substrate.

In FIG. 54, a substantially monochromatic light beam emitted from alight source LS such as an He—Ne an laser, a semiconductor laser or abright line lamp enters beam splitter systems 156A and 156B through abeam shaping device 150 for adjusting the diameter of the beam to apredetermined size, an aperture 152 for shaping the cross-sectionalshape of the beam into a shape substantially similar to the generalshape of a diffraction grating mark MG on a wafer W, and a lens system154. Further, the irradiating beam LB transmitted through the beamsplitter systems is transmitted through a lens system 158 equal to thelens system G2 in FIG. 49 and arrives at the grating mark MG formed onthe wafer W on a stage WST.

The aperture 152 and the surface of the wafer W (or a fiducial markplate FG) are set conjugately with each other with respect to thecomposite system of the lens system 154 and an objective lens system158. Accordingly, the irradiating beam LB transmitted through theaperture 152 is set to a parallel light beam, whereby the light beam LBarriving at the wafer W also becomes a parallel light beam. Also, in thecase of FIG. 54, it is to be understood that the irradiating light beamLB enters the surface of the wafer (the surface of the fiducial markplate) perpendicularly thereto.

In FIG. 54, the coordinates position of the stage WST moved with thewafer W, the fiducial mark plate FG and a movable mirror 43 placedthereon is measured by a laser interferometer 44 as in FIG. 49, and themovement of the stage WST is effected by a driving system 42, which iscontrolled basically by a servo control system 64 as in FIG. 52.

Now, diffracted lights of respective orders such as ±1st-orderdiffracted beams ±D01 and ±2nd-order diffracted beams ±D02 are createdfrom the grating mark MG (the grating on the fiducial mark plate FG)irradiated with the irradiating light beam LB. Of these, the 1st-orderdiffracted beams ±D01 are transmitted through the objective lens system158, are reflected by reflecting surfaces Mra and Mrb formed on portionsof the beam splitter system 156A and enter the lens system 160,whereafter they are converted into parallel light beams by this lenssystem 160 and intersect each other at a predetermined angle on areference grating plate 162.

Thereby, one-dimensional interference fringes by the interferencebetween the two 1st-order diffracted beams ±D01 (the intensitydistribution of the diffracted images of the grating mark MG by the1st-order diffracted lights) are produced on the reference grating plate162. The pitch Pf1 of the light and shade of the interference fringes isPf1=Mm1 (Pmg/2) when it is represented by the relation between the imagemagnification Mm1 of an imaging optical system comprised of theobjective lens system 158 and the lens system 160 (the magnificationwhen the reference grating plate 162 is seen from the wafer W side) andthe pitch Pmg of the grating mark MG, and when the image magnificationMm1 is one time, the pitch Pf1 of the light and shape of theinterference fringes becomes just a half of the pitch Pmg of the gratingmark MG.

When a diffraction grating (duty 50%) of the transmission type having apitch equal to the pitch of the light and shade of the interferencefringes is formed on the reference grating plate 162, such transmittedbeams ±D11 of which the quantity of light becomes maximum when thetransmitting line portions of the diffraction grating and the light lineportions of the interference fringes coincide with each other and thequantity of light becomes minimum when the transmitting line portions ofthe diffraction grating and the dark line portions of the interferencefringes coincide with each other are received by a photoelectric elementDT11.

Thus, when the grating mark MG on the wafer W (i.e., the stage WST) isminutely moved in the pitch direction relative to the irradiating beamLB, the interference fringes produced on the reference grating plate 162also minutely move in the pitch direction, and the waveform of aphotoelectric signal I11 outputted from the photoelectric element DT11during that movement changes as shown in FIG. 56A. In FIG. 56A, the axisof ordinates represents the level of the photoelectric signal I11 andthe axis of abscissas represents the moved position of the grating markMG on the wafer W in the pitch direction (here, X-direction).

FIG. 56A shows changes in the level of the signal I11 obtainedparticularly when the grating mark MG and the irradiating beam LB aremoving at equal speed in X-direction relative to each other, and thesignal I11 assumes a substantially sine wave-like waveform of which oneperiod is the amount of movement of ½ of the pitch Pmg of the gratingmark MG. Also, it is to be understood that the amplitude (peak/peak)value of the signal I11 is E11.

On the other hand, in FIG. 54, 2nd-order diffracted beams ±D02 createdfrom the grating mark MG by the application of the irradiating beam LBenter the objective lens system 158, and thereafter are reflected by thepartial reflecting surfaces Mrc and Mrd of the beam splitter system 156Band arrive at the lens system 164. The two 2nd-order diffracted beams±D02 are converted into parallel light beams by the lens system 164 andare deflected so as to intersect each other on the reference gratingplate 166. Thereby, one-dimensional interference fringes by theinterference between the two 2nd-order diffracted beams ±D02 (theintensity distribution of the diffracted images of the grating mark MGby the 2nd-order diffracted lights) are produced on the referencegrating plate 166. The pitch Pf2 of the light and shade of theseinterference fringes is Pf2=Mm2 (Pmg/4) when it is represented by therelation between the image magnification Mm2 of an imaging opticalsystem comprised of the objective lens system 158 and the lens system164 (the magnification when the reference grating plate 166 is seen fromthe wafer W side) and the pitch Pmg of the grating mark MG, and when theimage magnification Mm2 is one time, the pitch Pf2 of the light andshade of the interference fringes becomes just ¼ of the pitch Pmg of thegrating mark MG.

When a diffraction grating (duty 50%) of the transmission type having apitch equal to the pitch Pf2 of the light and shade of the interferencefringes is formed on the reference grating plate 166, such transmittedbeams ±D22 of which the quantity of light becomes maximum when thetransmitting line portions of the diffraction grating and the light lineportions of the interference fringes coincide with each other and thequantity of light becomes minimum when the transmitting line portions ofthe diffraction grating and the dark line portions of the interferencefringes coincide with each other are received by a photoelectric elementDT22. Thus, when the grating mark MG on the wafer W (i.e., the stageWST) is minutely move in the pitch direction relative to the irradiatingbeam LB, the interference fringes produced on the reference gratingplate 166 also minutely move in the pitch direction.

The waveform of a photoelectric signal I22 outputted from thephotoelectric element DT22 during that movement changes as shown in FIG.56B. In FIG. 56B, the axis of ordinates represent the level of thephotoelectric signal I22 and the axis of abscissas represents the movedposition of the grating mark MG on the wafer W in the pitch direction(here, X-direction). FIG. 56B shows changes in the level of the signalI22 obtained particularly when the grating mark MG and the irradiatingbeam LB are moving at equal speed in X-direction relative to each other,and the signal I22 assumes a substantially sine wave-like waveform ofwhich one period is the amount of movement of ¼ of the pitch Pmg of thegrating mark MG. Also, it is to be understood that the amplitude(peak/peak) value of the signal I22 is E22.

Now, assuming that in the waveforms of the signals shown in FIGS. 56Aand 56B, the grating mark to be position-detected, like the diffractiongrating on the fiducial mark plate FG, is free of any change in theamplitude reflectance for each of 1st-order diffraction and 2nd-orderdiffraction and the reference grating plates 162 and 166 are ideallydisposed without any error in the relative positional relationtherebetween, each of the peak point position and the bottom pointposition on the signal I11 accurately coincides with each peak pointposition on the signal I22. So, by measuring particular peak pointpositions X11 and X22 on the signals I11 and I22 on the basis of therelation between each change in the levels of the signals I11 and I22.and the moved position of the stage WST, the position of the gratingmark MG is detected.

As a portion of a signal processing circuit therefor, provision is madeof a signal waveform sampling circuit as shown in FIG. 55. In FIG. 55,the signals I11 and 122 are amplified by a predetermined amount byamplifiers 180A and 180B, respectively, and thereafter are sampled byA/D converter circuit 182A and 182B, respectively. A clock signal Cpsfor determining the timing of the sampling is made from the count pulseof the interferometer 44 through a synchronous logic circuit 184. Thevalues of the levels of the signals I11 and I22 sampled by the A/Dconverter circuits 182A and 182B, respectively, are stored in memorycircuits 186A and 186B in the order of addresses. In that case, theaddress data of the memory circuits 186A and 186B during writing areproduced from the synchronous logic circuit 184 on the basis of thecount pulse of the interferometer.

Thus, the waveform data of the signals I11 and I22 are stored in thememory circuits 186A and 186B while the stage WST is being moved over apredetermined scanning range, and the respective waveform data becomesimilar to ones in which in FIGS. 56A and 56B, the axes of abscissas aremade to primarily correspond to the address values of the memorycircuits. In that case, assuming that the minimum count of theinterferometer 44 corresponds to 0.02 μm of the movement of the stage,an address change of the memory circuits 186A and 186B corresponds to0.02 μm. The waveform data stored in the memory circuits 186A and 186Bare then read into a processing circuit equal to the position offsetamount detection circuit 56 or the amplitude ratio detection circuit 58shown in FIG. 52, and the detection of the position of the grating markMG is effected.

In that case, the position offset amount detection circuit 56 reads outthe waveform data of the signal I11 stored in the memory circuit 186A(the addresses thereof and the coordinates position of the stage WST inX-direction correspond to each other at one to one) and determines theparticular peak point position X11 shown in FIG. 56A by calculation, andalso reads out the waveform data of the signal I22 stored in the memorycircuit 186B (the addresses thereof and the coordinates position of thestage WST in X-direction correspond to each other at one to one) anddetermines the particular peak point position X22 shown in FIG. 56B bycalculation. In the case of the grating mark MG on the wafer W, thosepositions X11 and X22 may become different depending on the asymmetry ofthe amplitude reflectance of the grating mark MG including the resistlayer.

Also, the amplituderatio detection circuit 58 reads out the waveforms ofthe signals stored in the memory circuits 186A and 186B and finds theamplitude values E11 and E22 as shown in FIGS. 56A and 56B. Then thefinal position X0 of the grating mark MG is found by a calculationcircuit (CPU) equal to the weighted mean circuit 60 in the processingcircuit shown in FIG. 52 on the basis of the following weighted meanhaving the amplitude of each signal as weight.

X 0=(E 11·X 11+E 22·X 22)/(E 11+E 22)

In the case of this arithmetic expression, the weight coefficients arethe amplitude values themselves of the signals I11 and I22, but asdescribed with respect to the previous twenty-first embodiment, theamplitude values K11 and K22 of the signals I11 and I22 obtained when anideal grating (the grating mark of the fiducial mark plate FG) having noasymmetry in its amplitude reflectance is detected may be found inadvance, and the ratios C11 and C22 between the amplitude values E11,E22 of the signals when the grating mark MG of the wafer is detected andthe amplitude values K11 and K22 may be made into

C 11=E 11=K 11/(K 11+K 22)

C 22=E 22·K 22/(K 11+K 22)

whereafter the final position X0′ of the grating mark may be calculatedby

X 0′=(C 11·X 11+C 22·X 22)/(C 11+C 22)

However, the expression-for finding this position X0′ can be calculatedby the following expression without the ratios C11 and C22 beingespecially calculated.

X 0′=(E 11·K 11·X 11+E 22·K 22·X 22)/(E 11·K 11+E 22·K 22)

The amplitude values K11 and K22 found here also correspond to thetheoretical intensity ratio between 1st-order diffracted light and2nd-order diffracted light, and assuming that the theoretical intensityof 2nd-order diffracted light when the intensity of 1st-order diffractedlight is 1 is β (β<1), the position X0′ having added thereto thetheoretical weight by the order is determined by the followingexpression:

X 0′=(E 11·X 11+E 22·β·X 22)/(E 11+E 22·β)

Now, in the present embodiment, in order to effect the detection of theposition of the mark by the homodyne system, it is simplest in signalprocessing to find out particular peak point positions (or bottom pointpositions) of the signal waveforms as shown in FIGS. 56A and 56B.However, to find the peak points on the signal waveforms, a calculatingprocess such as making a differentiated waveform obtained bydifferentiating original signal waveform data through a numerical valuefilter, and find the position of the zero point on the differentiatedwaveform is necessary. Therefore, it is also necessary to remove inadvance a weak noise component superposed on the original signalwaveform, by some technique. However, even in the homodyne system, it isalso possible to apply signal processing in which the noisecharacteristic is improved by phase difference measurement utilizing aFourier integral process similar to that in the heterodyne system.

So, a method when the phase difference calculating process is applied inthe homodyne system will now be described with reference to FIGS. 57Aand 57B. First, the waveform data of the signals I11 and I22 shown inFIGS. 56A and 56B are stored in the memory circuits 186A and 186B oversuitable periods (e.g. 10 periods or more). Thereafter, the integratedvalues Vrs and Vrc of each of reference sine wave data R1 (sin) andreference cosine wave data R1 (cos) in which a common position on thewaveform of the signal I11, e.g. a position X00 shown in FIGS. 57A and57B, is assumed as a reference phase point, and the waveform data of thesignal I11 are found, and a phase angle θ11 satisfying tan θ11=Vrs/Vrcis calculated from the two integrated values Vrs and Vrc.

Since one period of the signal I11 corresponds to ½ of the pitch Pmg ofthe grating mark MG, the phase angle θ11 is converted into a positionoffset amount ΔX11 relative to the reference position X00 found from thewaveform data of the signal I11 as shown in FIG. 57A.

The reference sine wave data R1 (sin) and the reference cosine wave dataR1 (cos) should desirably be stored in advance as a data table on thememory of the CPU. The data table is stored with a sine wave functionhaving ½ of the design pitch Pmg of the grating mark MG as a periodbeing produced by the same resolving power as that of the waveform datain the memory circuits 186A and 186B (a datum on the function coincideswith the sampling unit by the interferometer). Also, the referencecosine wave data R1 (cos) is made by shifting the read-out referencedata position (address) of the reference sine wave data R1 (sin) by π/2(Pmg/8).

Likewise, the integrated values Vrs and Vrc of each of reference sinewave data R2 (sin) and reference cosine wave data R2 (cos) in which acommon position X00 on the waveform of the signal I22 is assumed as thereference phase point and the waveform data of the signal I22 are found,and a phase angle θ22 satisfying tan θ22=Vrs/Vrc is calculated from thetwo integrated values Vrs and Vrc. Since one period of the signal I22corresponds to ¼ of the pitch Pmg of the grating mark MG, the phaseangle θ22 is converted into a position offset amount ΔX22 relative tothe reference position X00 found from the waveform data of the signalI22 as shown in FIG. 57B.

Also, the data table of the reference sine wave data R2 (sin) andreference cosine wave data R2 (cos) is stored with a sine wave functionhaving ¼ of the design pitch Pmg of the grating mark MG as a periodbeing produced by the same resolving power as that of the waveform datain the memory circuits 186A and 186B, and the reference cosine wave dataR2 (cos) is made by shifting the read-out data position (address) of thereference sine wave data R2 (sin) by π/2 (Pmg/16).

The position offset amounts ΔX11 and ΔX22 calculated in theabove-described manner assume the same value in an ideal case (such asthe fiducial mark) where the grating mark MG is free of the asymmetry ofthe amplitude reflectance, but depending on the degree of the asymmetry,as described above, the position of the grating mark detected on thebasis of 1st-order diffracted light alone and the position of thegrating mark detected on the basis of 2nd-order diffracted light alonediffer from each other. So, the calculated position offset amounts ΔX11and ΔX22 are weighted on the basis of the amplitudes E11 and E22 of thesignals I11 and I22 and the theoretical intensity ratio between1st-order diffraction and 2nd-order diffraction to thereby provideweighted mean, whereby the position offset amount ΔX00 from theapparently most certain reference position X00 of the grating mark isdetermined.

In this case, the reference position X00 is stored in the servo controlsystem 64 (or the CPU or the like) in FIG. 52 as the target coordinatesposition when the stage WST is positioned for wafer alignment. So, whenthe stage WST is to be positioned by the servo control system 64, thedriving system 42 can be servo-controlled so that the coordinatesposition in X-direction measured by the interferometer 44 may be(X00+ΔX00 or X00−ΔX00).

However, when alignment is to be done relative to a point discrete fromthe grating mark MG on the wafer W, for example, the central point onthe shot area of a mask exposed by a circuit device pattern, it is notalways necessary to move the stage WST so that the grating mark MG maybe aligned with the reference grating plates 162 and 166 in FIG. 54 andtherefore, the calculated position offset amount ΔX00 can be stored inthe CPU and the value thereof can be utilized for the position controlof the stage WST for the alignment (positioning) during the exposure ofthe shot area on the wafer.

Specifically, the reference position X00 is set as the targetcoordinates position (deviated by 1 μm or less relative to the trueposition) of the grating mark MG determined by the result of theprealignment or global alignment of the wafer W, whereafter the positionoffset amount ΔX00 of the. grating mark MG is detected. Thereafter, inorder to make the central point of the shot area coincident with theexposure center (the central point of a mask pattern), the targetcoordinates position of the stage WST determined by the result of theprealignment or global alignment is corrected and calculated by thecalculated position offset amount X00, whereby the stage WST can beservo-controlled relative to a calculated new target coordinatesposition.

As the system in which the grating mark MG on the wafer W is thusdetected to thereby effect alignment, there are a system in which thewafer is moved so as to precisely align the grating mark MG with thereference of the position detecting apparatus, and a system in which therelative position offset amount of the grating mark MG and the referenceof the position detecting apparatus is measured, whereafter the wafer ismoved in conformity with the position offset amount without the gratingmark MG being precisely aligned with the reference of the positiondetecting apparatus.

In the above-described embodiment, it has been to be understood that therelative position between the reference grating plates 162 and 166 inFIG. 54 in the pitch direction is precisely uniform, but in reality, theadjustment of the order of submicron is difficult even if the adjustmentin the manufacture of the apparatus is done elaborately. If a relativeposition error ΔXe remains between the reference grating plates 162 and166, this position error ΔXe will be included as the error amount whenthe position coordinates or the position offset amount of the gratingmark MG is found.

So, the position of the fiducial mark found by the photoelectricdetection of ±1st-order diffracted beams ±D01 and the position of thefiducial mark found by the photoelectric detection of ±2nd-orderdiffracted beams ±D02 are measured by the utilization of the gratingmark of the fiducial mark plate FG in FIG. 54, and the differencebetween those positions is calculated as the position error ΔXe and isstored as an offset value (broadly, a base line error) in the CPU whenthe grating mark MG on the wafer W is to be actually detected, the valueof one of the position X11 (FIGS. 56A and 56B) of the grating mark MGfound from ±1st-order diffracted beams and the position X22 (FIGS. 56Aand 56B) of the grating mark MG found from ±2nd-order diffracted beamscan be corrected by an amount corresponding to the stored position errorΔXe, and then can be calculation-processed so as to provide weightedmean.

Also, when as in FIGS. 57A and 57B, the position of the grating mark MGis to be detected from the position offset amount by phase measurement,an offset corresponding to the position error ΔXe is included betweenthe position offset amount ΔX11 determined on the basis of ±1st-orderdiffracted lights and the position offset amount ΔX22 determined on thebasis of ±2nd-order diffracted lights and therefore, for example, avalue found by subtracting the position error ΔXe from the apparentlymost certain position offset amount ΔX00 calculated by weighted meancalculation in the CPU can be used as a position offset amount.

Alternatively, inclinable plane parallel plate glass may be disposedforwardly of at least one of the reference grating plates 162 and 166,and provision may be provided such a mechanism as will minutely displacethe positions of ±1st-order diffracted beams ±D01 and ±2nd-orderdiffracted beams ±D02 incident on the reference grating plate 162 or 166in the pitch direction. In that case, the positions X11, X22 and theposition offset amounts ΔX11, ΔX22 of the grating mark of the fiducialmark plate FG in FIG. 54 are found by the detection of each of±1st-order diffracted lights and ±2nd-order diffracted lights,whereafter the inclination of the plane parallel plate glass can beautomatically adjusted so that those values may be X11=X22 or ΔX11=ΔX22.

Further, when the inclinable plane parallel plate glass is provided, theinclination of the plane parallel plate glass may be adjusted so as toroughly correct the position error ΔXe, whereafter any residual erroramount may be stored as an offset in the CPU.

A twenty-third embodiment in which the position detecting apparatusdescribed in each of the twenty-first and twenty-second embodiments isapplied as various alignment detection systems in a projection exposureapparatus will now be described with reference to FIGS. 15, 58, 17A and17B. Again in this embodiment, there is applied the schematicdisposition of the alignment system of a projection exposure apparatusas shown in FIG. 15 which requires the measurement of a base line amountfor determining the relative positional relation between the centralprojection point of a mask (reticle) mounted on the projection exposureapparatus and the detection center points of the various alignmentsystems.

When the position detecting apparatus described in each of thetwenty-first and twenty-second embodiments is applied to variousalignment systems, their detection center points are prescribed by thereference grating plate SG (FIG. 50) for producing a reference beatsignal when a mark position detecting apparatus of the heterodyne type,and are prescribed by the reference grating plates 162 and 166 (FIG. 54)for receiving ±1st-order diffracted beams and ±2nd-order diffractedbeams from the grating mark MG on the wafer when a mark positiondetecting apparatus of the homodyne type is applied.

The construction of the projection exposure apparatus of FIG. 15 hasalready been described and therefore, the details thereof need not bedescribed.

However, when a reticle alignment system RA is of a construction inwhich a mark (grating pattern) RM for reticle alignment around a reticleR and a corresponding grating mark on the fiducial mark plate FG areirradiated with illuminating light of the same wavelength asilluminating light for the projection exposure of a circuit pattern PRand a reticle stage RST is finely moved so that the two marks may assumea predetermined positional relation, a detection center point Rf1 is notrequired. This also holds true of an alignment system TTRA, and when therelative positional relation between the grating mark on the fiducialmark plate FG or the grating mark MG on the wafer W and a grating markfor die-by-die (D/D) alignment formed on the portion around the circuitpattern PR of the reticle R is to be directly detected, it isunnecessary to especially use a detection center point Rf2 (referencegrating plate).

FIG. 58 shows an example of the essential portion of an alignment systemTTLA of the alignment systems shown in FIG. 15, and a pair of lightsending beams ±LFs (corresponding to the beam ±LFs and beam −LFs in FIG.49) for detecting the grating mark MG on the wafer W or the fiducialmark plate FG by the heterodyne system shown in FIGS. 49 and 50 enter aprojection lens PL via a correction optical system CG, a polarizing beamsplitter PBS (functionally corresponding to the half mirror BS1 in FIG.49), a ¼ wavelength plate QW, an objective lens OBJ (corresponding tothe objective lens G2 in FIG. 49) and two mirrors MR. At this time, aplane FC conjugate with the surface of the wafer W is formed between thetwo mirrors MR, and the pair of beams ±LFs intersect each other in thisplane FC. The beams ±LFs are relayed by the projection lens PL, andintersect each other also on the wafer W and irradiate the grating markMG.

In the present embodiment, the beams ±LFs entering the polarizing beamsplitter PBS are linearly polarized light, and the light sending beamsefficiently reflected by the polarizing beam splitter PBS are convertedinto circularly polarized light rotating in one direction when they aretransmitted through the ¼ wavelength plate QW and pass through theprojection lens PL and irradiate the grating mark MG on the wafer W. Aninterference beam BM created vertically thereby from the grating mark MGpasses through substantially the center of the pupil plane EP of theprojection lens PL, and arrives at the polarizing beam splitter PBS viathe two mirrors MR, the objective lens OBJ and the ¼ wavelength plateQW. At this time, the interference beam BM of ±1st-order diffractedbeams and the interference beam of 0-order beam and ±2nd-order beam arelinearly polarized lights orthogonal to the direction of polarizationand are therefore efficiently transmitted through the polarizing beamsplitter PBS, and arrive at a photoelectric converter 36B provided withphotoelectric elements DT1, DT2 a and DT2 b similar to those in FIG. 49.

In such an alignment system TTLA, the wavelength of the light sendingbeams ±LFs (e.g. 633 nm of an He—Ne laser) is defined so as to beconsiderably longer than the wavelength of the illuminating light forexposure and thus, is subjected to the influence of the chromaticaberration (on the axis and magnification) of the projection lens PL andthe influence of the chromatic aberration of the objective lens OBJ. So,as shown in FIG. 58, a correction optical system CG for correcting anoptical error occurring in conformity with the chromatic aberration isprovided in the optical path of the light sending beams ±LFs. Thiscorrection optical system CG is comprised of a convex lens, a concavelens or a combination thereof, or plane parallel plate glass or thelike.

Also, in the case of the alignment system TTRA in FIG. 15, when a markDDM for D/D alignment on the reticle R is a diffraction grating and therelative position offset thereof with respect to the grating mark MG onthe wafer W corresponding to the mark DDM is to be detected by theheterodyne system as shown in FIGS. 49 and 50, a transparent planeparallel plate-like correction plate PGD can be provided on the pupilplane EP of the projection lens PL as is disclosed in (G) JapanesePatent Application Laid-Open No. 6-302504 (File No. 198,077, Feb. 17,1994), and a transmission type phase grating (one in which uneven linesare etched at a predetermined pitch on the surface of the correctionplate PGP) can be formed only at a position on the correction plate PGPthrough which the light sending beams (±LFs) and the interference beam(BM) pass to thereby reduce the influences of on-axis chromaticaberrations and chromatic difference of magnification.

FIGS. 59A-59D show the construction of a projection exposure apparatusaccording to a twenty-forth embodiment which incorporates such acorrection plate PGP in a portion of the alignment system TTRA, FIG. 59Abeing a view in which the optical paths of the light sending beams ±LFsand interference beam BM during the detection of the grating mark MGhaving a pitch in X-direction (measurement direction) are seen in X-Zplane, and FIG. 59B being a view in which the optical paths of FIG. 59Aare seen in Y-Z plane orthogonal thereto.

A pair of light sending beams ±LFs emerge from the objective lens OBJ(corresponding to the objective lens G2 in FIG. 49) of the alignmentsystem TTRA while being slightly eccentric from an optical axis AXa, arereflected by a mirror MR and enter the projection lens PL through thewindow RW around the pattern area of the reticle R. The pair of lightsending beams ±LFs, when seen in X-Z plane, are transmitted through thewindow RW with a symmetrical inclination as shown in FIG. 59A, and whenseen in Y-Z plane, are transmitted through the window RW while beinginclined with respect to the optical axis AXa of the objective lens OBJ,as shown in FIG. 59B.

The pair of light sending beams ±LFs pass through two phase typediffraction gratings (hereinafter referred to as the phase gratings)PG1S and PG2S, respectively, on the correction plate PGP disposed on thepupil plane EP of the projection lens PL. At this time, by the action ofthe phase gratings PGlS and PG2S, the respective light sending beams±LFs have their inclinations changed by a predetermined amount in apredetermined direction from a broken line in the figure as indicated bya solid line and emerge from the projection lens PL. The light sendingbeams ±LFs, when seen in X-Z plane, irradiate the grating mark MG on thewafer W at symmetrical angles, and when seen in Y-Z plane, enter thegrating mark MG while begin somewhat inclined., in Y-direction withrespect thereto.

The interference beam BM created thereby from the grating mark MG againenters the projection lens PL and on the pupil plane EP, it passesthrough a position differing from the phase gratings PG1S and PG2S. Atthat position, there is formed a phase grating PG3S for inclining theinterference beam BM by a predetermined amount in a predetermineddirection from a broken line in FIG. 59B as indicated by a solid line,whereby the optical path of the interference beam BM is corrected so asto be transmitted through the projection lens PL and toward the windowRW of the reticle R. The interference beam BM passed through the windowRW travels toward a light receiving system (G1, AP, BS1, G3, DT1, DT2)as shown in FIG. 49 via the mirror MR and the objective lens OBJ. Atthis time, the interference beam BM is transmitted through the window RWof the reticle R while being slightly inclined in the non-measurementdirection with respect to the optical axis AXa of the objective lensOBJ.

The use of such a correction plate PGP is of course possible also forthe alignment system TTLA shown in FIG. 58. For example, in the case ofan exposure apparatus using a projection lens (or a combination of areflecting mirror and a dioptric lens) of which the lens glass materialis quarts or fluorite and which uses ultraviolet rays of a wavelength of180-300 nm (such as an excimer laser beam) as exposure light, chromaticaberration for the wavelength of a beam from an He—Ne laser or asemiconductor laser becomes very great and the conjugate surface FC ofthe wafer shown in FIG. 58 becomes separate by several tens of cm fromthe projection lens. So, by the use of the correction plate PGP, theconjugate surface FC of the wafer on which the light sending beams ±LFsintersect each other is corrected so as become close to the projectionlens.

Also, it is possible to bring the optical paths of the light sendingbeams LFs and the detecting beam BM shown in FIGS. 59A and 59B into anentirely opposite relation, and design is made such that a light sendingbeam of a single wavelength is projected from the objective lens OBJalong the optical path of the detecting beam BM and irradiates thegrating mark MG on the wafer W through a phase grating PG3 in theprojection lens PL. Thereby, ±1st-order diffracted beams travelling backalong the same optical path as the light sending beams ±LFs in FIGS. 59Aand 59B are created from the grating mark MG, and these beams arrive atthe window RW of the reticle R through a phase grating PG1 in theprojection lens PL, and are transmitted therethrough and return to theobjective lens OBJ.

Further, ±2nd-order diffracted beams are also created from the gratingmark MG at an angle of diffraction greater than that of ±1st-orderdiffracted beams. The ±2nd-order diffracted beams pass through positionson the pupil plane EP (correction plate PGP) of the projection lens PLseparated in the pitch direction of the grating mark MG relative to thepassage positions of ±1st-order diffracted beams. So, besides the phasegrating PG3S passing a single light sending beam therethrough and thephase gratings PG1S and PG2S passing therethrough the ±1st-orderdiffracted beams from the grating mark MG, a correction plate PGP formedwith phase gratings PG4S and PG5S passing therethrough the ±2nd-orderdiffracted beams from the grating mark MG is disposed on the pupil planeEP.

If this is done, ±1st-order diffracted beams and ±2nd-order diffractedbeams created from the grating mark MG can all be returned to theobjective lens OBJ through the window RW of the reticle R, and thealignment system of the homodyne type shown in FIG. 54 becomesutilizable.

Further, when the light sending beam is made single and the alignmentsystem of the homodyne type is assembled by the use of the correctionplate PGP as shown in FIGS. 59A, 59B and 60, an interference image(diffracted image) reflecting the position offset of the grating mark MGcan be produced in a plane on which the diffracted beam of each orderfrom the grating mark MG on the wafer W is imaged and therefore, designmay be made such that the interference image is picked up by aone-dimensional or two-dimensional CCD camera or the like and the imagesignal thereof is analyzed to thereby detect the position of the gratingmark MG relative to a reference position (for example, a particularpixel position in the image pickup field of the CCD camera or an indexmark disposed in an alignment optical path).

In this case, it is premised that the wafer stage is stationary while atleast the image signal is introduced into a signal processing circuit,but also is also a case where under outside influence, the stage WST isvibrating when observed in the order of nanometer. So, it is desirableto prepare two CCD cameras so that the interference image produced by±1st-order diffracted beams from the grating mark MG and theinterference image produced by ±2nd-order diffracted beams may be readinto a signal processing circuit (particularly a waveform memory)individually by the respective CCD cameras and at the same timing.Alternatively, provision is made of a prism or the like for minutelyinclining the ±1st-order diffracted beams and ±2nd-order diffractedbeams from the grating mark MG in a direction orthogonal to the pitch ofthe grating mark near the Fourier transform plane in the alignmentsystem so that the interference image of ±1st-order diffracted beams andthe interference image of ±2nd-order diffracted beams produced on theimage pickup surface of two-dimensional CCD cameras may be shifted in adirection orthogonal to the pitch of the grating mark MG and theinterference images may be picked up at a time by a single CCD camera.

Again in a case where the position or the position offset amount of thegrating mark MG is found by such image detection, the interference imageby the ±1st-order diffracted beams from the grating mark MG and theinterference image by the ±2nd-order diffracted beams can be picked updiscretely from each other and the position offset amount or the likecan be individually found and then, as in the above-describedembodiments, the apparently most certain mark position offset amount (ormark position) can be determined by means calculation having addedthereto weight conforming to the amplitude value or the like of eachinterference image. Of course, again in this case, the phase differencemeasurement by the Fourier integral method described with reference toFIGS. 57A and 57B can be applied.

As described above, in each embodiment of the present invention, thediffracted lights photoelectrically detected are ±1st-order diffractedbeams and ±2nd-order diffracted beams from the grating mark MG, whereasthe orders of the diffracted lights detected are not limited thereto,but any combination such as a combination of ±1st-order diffracted beamsand ±3rd-order diffracted beams or a combination of ±2nd-orderdiffracted beams and ±5th-order diffracted beams can be chosen. Further,the combination of the diffracted beams of different orders used forposition detection or position offset detection is not limited to twosets as in each embodiment, but diffracted beams of three or more setsof different orders (e.g. ±1st-order, ±2nd-order and ±3rd-orderdiffracted beams) may be photoelectrically detected, and the markposition or the position offset amount found for each order is subjectedto weighted mean as in each previous embodiment to thereby determine theapparently most certain mark position and position offset amount.

As described above, according to the twenty-first to twenty-fourthembodiments, diffracted lights of different orders created correspondingto the plurality of spatial frequency components of a mark for positiondetection having periodic structure are photoelectrically detected, andon the basis of the photoelectric signals, a mark position (positionoffset) corresponding to each spatial frequency component isindividually found and further, weight accompanying an amplitude changecorresponding to the change in the intensity of the diffracted light ofeach order is applied to thereby average and therefore, there is theeffect that the irregularity of an error occurring with the markposition detection for each spatial frequency component becomes smalldue to the averaging effect.

Further, the amplitude (the change in the intensity of the diffractedlight of each order) of the photoelectric signal obtained for eachspatial frequency component is used as the weight during weighted meansand therefore, there is also the advantage that when the change in theintensity (amplitude) of the diffracted light of an order correspondingto a certain spatial frequency component becomes extremely small, thereis little or no weight in the result of the detection of the markposition (position offset) based on the diffracted light of that orderand the bad influence there of on the final result of the detection issmall.

Also, since the orders of the diffracted lights to be detected differfrom each other, design is made such that the theoretical intensityratios conforming to those orders are added to weighted mean andtherefore, the influence of the intensity difference occurring inprinciple can also be corrected and more highly accurate positiondetection becomes possible.

What is claimed is:
 1. An exposure apparatus which exposes a substratewith a pattern formed on a mask, comprising: a projection optical systemwhich projects the pattern formed on the mask onto the substrate; and aposition detecting system which detects a mark formed on the substratenot through the projection optical system, wherein the positiondetecting system includes: a beam illuminating system which is arrangedat a predetermined side with respect to said substrate, and whichirradiates an illumination beam perpendicularly onto said mark; a firstdetecting system which is arranged at the same side as saidpredetermined side with respect to said substrate, the first detectingsystem including a first optical element which causes ±nth-order (n is anatural number) diffracted beams generated from said mark irradiatedwith said illumination beam to respectively incline at a predeterminedangle on a first reference grating and a first photoelectric elementwhich detects a beam from the first reference grating irradiated withthe ±nth-order diffracted beams; and a second detecting system which isarranged at the same side as said predetermined side with respect to thesubstrate, the second detecting system including a second opticalelement which causes ±mth-order (m is a natural number other than n)diffracted beams generated from said mark irradiated with saidillumination beam to respectively incline at a predetermined angle on asecond reference grating and a second photoelectric element whichdetects a beam from the second reference grating irradiated with the±nth-order diffracted beams.
 2. The exposure apparatus according toclaim 1, further comprising: a calculator which is electricallyconnected to said first and second photoelectric elements and calculatespositional information of said mark based on results of detection bysaid photoelectric elements.
 3. The exposure apparatus according toclaim 1, wherein said first and second reference gratings include atransmission type grating.
 4. The exposure apparatus according to claim1, wherein the number m is a natural number greater than the number n.5. The exposure apparatus according to claim 4, wherein a pitch of saidfirst reference grating is different from a pitch of said secondreference grating.
 6. The exposure apparatus according to claim 5,wherein the pitch of said first reference grating is greater than thepitch of said second reference grating.
 7. The exposure apparatusaccording to claim 4, wherein the number n is 1 and the number m is 2.8. A position detecting device which detects positional information of amark formed on a substrate, comprising: a beam illuminating system whichis arranged at a predetermined side with respect to the substrate, andwhich irradiates an illumination beam perpendicularly onto said mark; afirst detecting system which is arranged at the same side as saidpredetermined side with respect to said substrate, the first detectingsystem including a first optical element which causes ±nth-order (n is anatural number) diffracted beams generated from said mark irradiatedwith said illumination beam to respectively incline at a predeterminedangle on a first reference grating and a first photoelectric elementwhich detects a beam from the first reference grating irradiated withthe ±nth-order diffracted beams; a second detecting system which isarranged at the same side as said predetermined side with respect tosaid substrate, the second detecting system including a second opticalelement which causes ±mth-order (m is a natural number other than n)diffracted beams generated from said mark irradiated with saidillumination beam to respectively incline at a predetermined angle on asecond reference grating and a second photoelectric element whichdetects a beam from the second reference grating irradiated with the±mth-order diffracted beams; and a calculator which is electricallyconnected to said first and second photoelectric elements and calculatessaid positional information of said mark based on results of detectionby said photoelectric elements.
 9. The device according to claim 8,wherein the number m is a natural number greater than the number n, andsaid first reference grating has a first pitch, and said secondreference grating has a second pitch that is smaller than the firstpitch.
 10. The exposure apparatus according to claim 9, wherein saidfirst and second reference gratings include a transmission type grating.11. A position detecting method for detecting positional information ofa mark formed on a substrate, comprising: irradiating an illuminationbeam perpendicularly onto said mark; causing ±nth-order (n is a naturalnumber) diffracted beams generated from said mark irradiated with saidillumination beam to respectively incline at a predetermined angle on afirst reference grating, and detecting by a first photoelectric elementa beam from the first reference grating irradiated with the ±nth-orderdiffracted beams; and causing ±mth-order (m is a natural number otherthan n) diffracted beams generated from said mark irradiated with saidillumination beam to respectively incline at a predetermined angle on asecond reference grating, and detecting by a second photoelectricelement different from said first photoelectric element a beam from thesecond reference grating irradiated with the ±nth-order diffracted beam.12. The method according to claim 11, further comprising, calculatingsaid positional information of said mark based on results of detectionby said photoelectric elements.
 13. The method according to claim 11,wherein the number m is a natural number greater than the number n, andsaid first reference grating has a first pitch, and said secondreference grating has a second pitch that is smaller than the firstpitch.
 14. An exposure method comprising: aligning the substrate basedon positional information which is detected using a position detectingmethod according to claim 12; and projection exposing said alignedsubstrate with a pattern formed on a mask.
 15. A device manufacturingmethod comprising: exposing the substrate with a device pattern formedon the mask using an exposure method according to claim
 14. 16. Aposition detecting device which detects positional information of a markformed on a substrate, comprising: a beam illuminating system which isarranged at a predetermined side with respect to the substrate, andwhich irradiates a first illumination beam having a first wavelength anda second illumination beam having a second wavelength different from thefirst wavelength perpendicularly onto said mark; a detecting systemwhich is arranged at the same side as said predetermined side withrespect to said substrate, the detecting system including an opticalelement which causes diffracted beams generated from said markirradiated with said first illumination beam or said second illuminationbeam to respectively incline at a predetermined angle on a firstreference grating and a photoelectric element which detects a beam fromthe reference grating irradiated with the diffracted beams; and acalculator which is electrically connected to said photoelectric elementand calculates said positional information of said mark based on resultsof detection by said photoelectric element.
 17. The device according toclaim 16, wherein said photoelectric element includes a firstphotoelectric element which detects the diffracted beams generated fromsaid mark irradiated with said first illumination beam and a secondphotoelectric element which is different from the first photoelectricelement and detects the diffracted beams generated from said markirradiated with said second illumination beam.
 18. The device accordingto claim 17, wherein said detecting system includes an optical memberwhich leads the diffracted beams generated from the mark with the firstillumination beam to the first photoelectric element and leads thediffracted beams generated from the mark with the second illuminationbeam to the second photoelectric element.
 19. The device according toclaim 18, wherein said optical member has a function of wavelengthselection.
 20. The device according to claim 17, wherein said beamilluminating system irradiates said first and second illumination beamssimultaneously.
 21. A position detecting method for detecting positionalinformation of a mark formed on a substrate, comprising: irradiating afirst illumination beam having a first wavelength and a secondillumination beam having a second wavelength different from the firstwavelength perpendicularly onto said mark with respect to a periodicdirection of said mark; causing diffracted beams generated from saidmark irradiated with said first illumination beam or said secondillumination beam to respectively incline at a predetermined angle on areference grating; and detecting by a photoelectric element a beam fromthe reference grating irradiated with the diffracted beams, andcalculating said positional information of said mark based on thedetection by the photoelectric element.
 22. A position detecting methodfor detecting positional information of a mark formed on a substrate,comprising: irradiating a first illumination beam having a firstwavelength and a second illumination beam having a second wavelengthdifferent from the first wavelength perpendicularly onto said mark;causing diffracted beams generated from said mark irradiated with saidfirst illumination beam or said second illumination beam to respectivelyincline at a predetermined angle on a reference grating; and detectingby a photoelectric element a beam from the reference grating irradiatedwith the diffracted bemas, and calculating said positional informationof said mark based on the results of detection by the photoelectricelement.
 23. The method according to claim 22, wherein saidphotoelectric element includes a first photoelectric element whichdetects the diffracted beams generated from said mark irradiated withsaid first illumination beam and a second photoelectric element which isdifferent from the first photoelectric element and detects thediffracted beams generated from said mark irradiated with said secondillumination beam.
 24. The method according to claim 23, wherein saidfirst and second illumination beams are irradiated simultaneously. 25.An exposure method comprising: aligning the substrate based onpositional information which is detected by using a position detectingmethod according to claim 23; and projection exposing said alignedsubstrate with a pattern formed on a mask.
 26. A device manufacturingmethod comprising: exposing the substrate with a device pattern formedon the mask using an exposure method according to claim
 25. 27. Anapparatus according to claim 1, further comprising: a stage on whichsaid substrate is provided to move said mark relative to saidillumination beam in said periodic direction in order to detectpositional information of said mark.
 28. A method according to claim 13,wherein said ±nth-order and ±mth-order diffracted beams are generatedfrom said mark moved relative to said illumination beam in said periodicdirection.
 29. An exposure method comprising: exposing with an exposurebeam irradiated on a mask the substrate moved based on the positionalinformation of said mark detected by using a position detecting methodaccording to claim 11; and said mark is moved relative to saidillumination beam in said periodic direction to enable detecting thepositional information.
 30. An exposure apparatus comprising: a positiondetecting device according to claim 16; a projection optical systemarranged at the same side as the position detecting device with respectto the substrate to project a pattern of a mask onto the substrate; anda stage on which said substrate is provided to enable detecting thepositional information by movement of said mark relative to said firstand second illumination beams in said periodic direction and to enabletransfer of the projected pattern onto the substrate by moving the stagebased on the detected positional information.
 31. An apparatus accordingto claim 30, wherein said second illumination beam has the secondwavelength greater than 650 nm.
 32. An apparatus according to claim 31,wherein said second illumination beam has the second wavelength greaterthan 700 nm.
 33. An apparatus according to claim 32, wherein said secondillumination beam has the second wavelength greater than 750 nm.
 34. Anexposure method comprising: exposing with an exposure beam irradiated ona mask the substrate moved based on the positional information of saidmark detected by using a position detecting method according to claim22; and said mark is moved relative to said first and secondillumination beams in said periodic direction to enable detecting thepositional information.
 35. A method according to claim 34, wherein saidsecond illumination beam has the second wavelength greater than 650 nm.36. The device according to claim 16, wherein said reference gratingincludes a transmission type grating.